diff --git a/.gitignore b/.gitignore
index c061dd8..b44d593 100644
--- a/.gitignore
+++ b/.gitignore
@@ -25,3 +25,4 @@ go.work.sum
 # Custom
 /_output
 /deployment/db_data
+/tmp/**
diff --git a/api/openapi/openapi.yaml b/api/openapi/openapi.yaml
index f4eb877..4b0b222 100644
--- a/api/openapi/openapi.yaml
+++ b/api/openapi/openapi.yaml
@@ -39,7 +39,7 @@ tags:
   - name: user
 
 paths:
-  /signup:
+  /user/signup:
     post:
       tags:
         - user
diff --git a/docs/.keep b/docs/.keep
deleted file mode 100644
index e69de29..0000000
diff --git a/docs/error_code.md b/docs/error_code.md
new file mode 100644
index 0000000..12a3bbc
--- /dev/null
+++ b/docs/error_code.md
@@ -0,0 +1,7 @@
+# Platform level error code design
+
+- InternalError
+- InvalidParameter
+- AuthFailure
+- ResourceNotFound
+- FailedOperation
diff --git a/internal/howmuch/adapter/controller/user.go b/internal/howmuch/adapter/controller/user.go
index ccb80bf..4d650a9 100644
--- a/internal/howmuch/adapter/controller/user.go
+++ b/internal/howmuch/adapter/controller/user.go
@@ -25,33 +25,66 @@ package controller
 import (
 	"net/http"
 
+	"git.vinchent.xyz/vinchent/howmuch/internal/howmuch/model"
+	"git.vinchent.xyz/vinchent/howmuch/internal/howmuch/usecase/usecase"
 	"git.vinchent.xyz/vinchent/howmuch/internal/pkg/core"
+	"git.vinchent.xyz/vinchent/howmuch/internal/pkg/errno"
+	"github.com/gin-gonic/gin"
 )
 
 // User is the user controller interface, it describes all the handlers
 // that need to be implemented for the /user endpoint
 type User interface {
 	Signup(core.Context)
-	UpdateInfo(core.Context)
-	Login(core.Context)
-	Logout(core.Context)
-	ChangePassword(core.Context)
+	UpdateInfo(*gin.Context)
+	Login(*gin.Context)
+	Logout(*gin.Context)
+	ChangePassword(*gin.Context)
 }
 
-type UserController struct{}
+type UserController struct {
+	userUsecase usecase.User
+}
+
+var UserParamsErr = &errno.Errno{
+	HTTP:    http.StatusBadRequest,
+	Code:    errno.ErrorCode(errno.InvalidParameterCode, "UserParamsErr"),
+	Message: "user info is not correct",
+}
+
+func NewUserController(us usecase.User) User {
+	return &UserController{
+		userUsecase: us,
+	}
+}
 
 func (uc *UserController) Signup(ctx core.Context) {
-	ctx.JSON(http.StatusOK, "hello")
+	var params model.User
+
+	if err := ctx.Bind(&params); err != nil {
+		core.WriteResponse(ctx, UserParamsErr, nil)
+		return
+	}
+
+	// TODO: check params validity (govalidator)
+
+	_, err := uc.userUsecase.Create(ctx, &params)
+	if err != nil {
+		core.WriteResponse(ctx, err, nil)
+		return
+	}
+
+	core.WriteResponse(ctx, errno.OK, nil)
 }
 
-func (uc *UserController) UpdateInfo(ctx core.Context) {
+func (uc *UserController) UpdateInfo(ctx *gin.Context) {
 }
 
-func (uc *UserController) Login(ctx core.Context) {
+func (uc *UserController) Login(ctx *gin.Context) {
 }
 
-func (uc *UserController) Logout(ctx core.Context) {
+func (uc *UserController) Logout(ctx *gin.Context) {
 }
 
-func (uc *UserController) ChangePassword(ctx core.Context) {
+func (uc *UserController) ChangePassword(ctx *gin.Context) {
 }
diff --git a/internal/howmuch/adapter/repo/user.go b/internal/howmuch/adapter/repo/user.go
index df23842..80a48bc 100644
--- a/internal/howmuch/adapter/repo/user.go
+++ b/internal/howmuch/adapter/repo/user.go
@@ -34,9 +34,6 @@ import (
 	"github.com/jackc/pgx/v5/pgtype"
 )
 
-// Get business service model, convert to the DB model (generated by sqlc)
-// To test repo's methods, I have to mock a pgx.Conn
-
 type userRepository struct {
 	db *pgx.Conn
 }
@@ -67,9 +64,9 @@ func (ur *userRepository) Create(
 		UpdatedAt: pgtype.Timestamp{Time: time.Now(), Valid: true},
 	}
 
-	tx, ok := transaction.(*pgx.Conn)
+	tx, ok := transaction.(pgx.Tx)
 	if !ok {
-		return nil, errors.New("transaction is not a *pgx.Conn")
+		return nil, errors.New("transaction is not a pgx.Tx")
 	}
 
 	queries := sqlc.New(tx)
diff --git a/internal/howmuch/infra/router/router.go b/internal/howmuch/infra/router/router.go
index 540dc1d..bfcecf5 100644
--- a/internal/howmuch/infra/router/router.go
+++ b/internal/howmuch/infra/router/router.go
@@ -48,7 +48,13 @@ func Routes(engine *gin.Engine, c controller.AppController) *gin.Engine {
 		core.WriteResponse(ctx, errno.PageNotFoundErr, nil)
 	})
 
-	engine.POST("/signup", func(ctx *gin.Context) { c.User.Signup(ctx) })
+	v1 := engine.Group("/v1")
+	{
+		userV1 := v1.Group("/user")
+		{
+			userV1.POST("/signup", func(ctx *gin.Context) { c.User.Signup(ctx) })
+		}
+	}
 
 	return engine
 }
diff --git a/internal/howmuch/registry/user.go b/internal/howmuch/registry/user.go
index 036e1d6..776a03f 100644
--- a/internal/howmuch/registry/user.go
+++ b/internal/howmuch/registry/user.go
@@ -22,9 +22,14 @@
 
 package registry
 
-import "git.vinchent.xyz/vinchent/howmuch/internal/howmuch/adapter/controller"
+import (
+	"git.vinchent.xyz/vinchent/howmuch/internal/howmuch/adapter/controller"
+	"git.vinchent.xyz/vinchent/howmuch/internal/howmuch/adapter/repo"
+	"git.vinchent.xyz/vinchent/howmuch/internal/howmuch/usecase/usecase"
+)
 
 // NewUserController returns a user controller's implementation
 func (r *registry) NewUserController() controller.User {
-	return &controller.UserController{}
+	u := usecase.NewUserUsecase(repo.NewUserRepository(r.db), repo.NewDBRepository(r.db))
+	return controller.NewUserController(u)
 }
diff --git a/internal/howmuch/usecase/usecase/user.go b/internal/howmuch/usecase/usecase/user.go
index ca5c46f..76bc9f9 100644
--- a/internal/howmuch/usecase/usecase/user.go
+++ b/internal/howmuch/usecase/usecase/user.go
@@ -51,6 +51,10 @@ func (uuc *userUsecase) Create(ctx context.Context, u *model.User) (*model.User,
 	data, err := uuc.dbRepo.Transaction(
 		ctx,
 		func(txCtx context.Context, tx interface{}) (interface{}, error) {
+			// TODO: should check if the user exists
+			// DB will return an error since we have set email to UNIQUE.
+			// But we may not want to expose the exact db error.
+
 			u, err := uuc.userRepo.Create(txCtx, tx, u)
 			if err != nil {
 				return nil, err
@@ -68,11 +72,12 @@ func (uuc *userUsecase) Create(ctx context.Context, u *model.User) (*model.User,
 			return u, err
 		},
 	)
-	user := data.(*model.User)
-
 	if err != nil {
+		// TODO: We should wrap the error at service level
 		return nil, err
 	}
 
+	user := data.(*model.User)
+
 	return user, nil
 }
diff --git a/internal/pkg/core/context.go b/internal/pkg/core/context.go
index 832d46e..3d15dde 100644
--- a/internal/pkg/core/context.go
+++ b/internal/pkg/core/context.go
@@ -22,6 +22,18 @@
 
 package core
 
+import "time"
+
 type Context interface {
+	// Context
+	Deadline() (deadline time.Time, ok bool)
+	Done() <-chan struct{}
+	Err() error
+	Value(key any) any
+
+	// Request
+	Bind(obj any) error
+
+	// Response
 	JSON(code int, obj any)
 }
diff --git a/internal/pkg/errno/code.go b/internal/pkg/errno/code.go
index 23b0a7c..e332767 100644
--- a/internal/pkg/errno/code.go
+++ b/internal/pkg/errno/code.go
@@ -24,18 +24,28 @@ package errno
 
 import "net/http"
 
+type PlatformLevelErrCode string
+
+const (
+	InternalErrorCode    = "InternalError"
+	InvalidParameterCode = "InvalidParameter"
+	AuthFailureCode      = "AuthFailure"
+	ResourceNotFoundCode = "ResourceNotFound"
+	FailedOperationCode  = "FailedOperation"
+)
+
 var (
 	OK = &Errno{HTTP: http.StatusOK, Code: "", Message: ""}
 
 	InternalServerErr = &Errno{
 		HTTP:    http.StatusInternalServerError,
-		Code:    "InternalError",
+		Code:    InternalErrorCode,
 		Message: "Internal server error",
 	}
 
 	PageNotFoundErr = &Errno{
 		HTTP:    http.StatusNotFound,
-		Code:    "ResourceNotFound.PageNotFound",
+		Code:    ErrorCode(ResourceNotFoundCode, "PageNotFound"),
 		Message: "Page not found",
 	}
 )
diff --git a/internal/pkg/errno/errno.go b/internal/pkg/errno/errno.go
index 25fa3c8..ddc4afe 100644
--- a/internal/pkg/errno/errno.go
+++ b/internal/pkg/errno/errno.go
@@ -28,6 +28,10 @@ type Errno struct {
 	Message string
 }
 
+func ErrorCode(platformErrCode string, resourceErrCode string) string {
+	return platformErrCode + "." + resourceErrCode
+}
+
 // Error implements Error() method in error interface
 func (err *Errno) Error() string {
 	return err.Message
diff --git a/tmp/build-errors.log b/tmp/build-errors.log
deleted file mode 100644
index 0e06a41..0000000
--- a/tmp/build-errors.log
+++ /dev/null
@@ -1 +0,0 @@
-exit status 2exit status 2exit status 2exit status 2exit status 2
\ No newline at end of file