How do I SET the GOPATH environment variable on Ubuntu? What file must I edit?

I am trying to do a go get:

go get github.com/go-sql-driver/mysql

and it fails with the following error:

package github.com/go-sql-driver/mysql: cannot download, $GOPATH not set. For more details see: go help gopath

when I do a go env , a list of Go values is shown as below:

ubuntu@ip-xxx-x-xx-x:~$ go env
GOARCH="amd64"
GOBIN=""
GOCHAR="6"
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH=""
GORACE=""
GOROOT="/usr/lib/go"
GOTOOLDIR="/usr/lib/go/pkg/tool/linux_amd64"
CC="gcc"
GOGCCFLAGS="-g -O2 -fPIC -m64 -pthread"
CGO_ENABLED="1"

clearly the GOPATH is not set, how and where do I set it?

I see many threads that mention this error but none that provide an answer to my question, which file needs to be edited to provide a value for this path?