[import] How to use custom packages

I'm trying to create and use a custom package in Go. It's probably something very obvious but I cannot find much information about this. Basically, I have these two files in the same folder:

mylib.go

package mylib

type SomeType struct {

}

main.go

package main

import (
    "mylib"
)

func main() {

}

When I try to go run main.go, I get this error:

main.go:4:2: import "mylib": cannot find package

I've tried to run go build mylib.go first but it doesn't seem to be doing anything (no file generated, no error message). So any idea how I could do this?

I am an experienced programmer, but, quite new into Go world ! And I confess I've faced few difficulties to understand Go... I faced this same problem when trying to organize my go files in sub-folders. The way I did it :

GO_Directory ( the one assigned to $GOPATH )

GO_Directory //the one assigned to $GOPATH
__MyProject
_____ main.go
_____ Entites
_____ Fiboo // in my case, fiboo is a database name
_________ Client.go // in my case, Client is a table name

On File MyProject\Entities\Fiboo\Client.go

package Fiboo

type Client struct{
    ID int
    name string
}

on file MyProject\main.go

package main

import(
    Fiboo "./Entity/Fiboo" 
)

var TableClient  Fiboo.Client

func main(){
    TableClient.ID = 1
    TableClient.name = 'Hugo'

    // do your things here
}

( I am running Go 1.9 on Ubuntu 16.04 )

And remember guys, I am newbie on Go. If what I am doing is bad practice, let me know !

For this kind of folder structure:

main.go
mylib/
  mylib.go

The simplest way is to use this:

import (
    "./mylib"
)

I try so many ways but the best I use go.mod and put

module nameofProject.com

and then i import from same project I use

import("nameofProject.com/folder")

It's very useful to create project in any place

another solution:
add src/myproject to $GOPATH.

Then import "mylib" will compile.

For a project hosted on GitHub, here's what people usually do:

github.com/
  laike9m/
    myproject/
      mylib/
        mylib.go
        ...
      main.go

mylib.go

package mylib

...

main.go

import "github.com/laike9m/myproject/mylib"

...