Start off by creating your layout page with 2 x Edittext boxes and one or two buttons. As you are a beginner you should (just for test purposes) start off by hard coding a program with 2 test logins, one for seller and one for buyer.
Once you have got your test project working, your next step should be to learn how to incorporate an SQLite database into your test project. You can either watch the SQLite tutorial on just read and learn from posts on this forum, it's up to you though to learn how B4A actually works...