What to do after proposing a girl?

Listening to her answer.

After that though, it's time for a lot of talking, figuring out where you will live, where and when the wedding should be, what sort of wedding you want, etc etc etc.

If you are religious, go talk to your church leader. They will have a lot of advice and instruction for you.

If you're not religious then there are usually some marriage counseling services available for pre-marriage-couples. They're great and very helpful.