1. Without knowing how long it fermented and how long it bottle-conditioned, it's impossible to answer your why-does-it-taste-yeasty question.
2. As mentioned, read John Palmer's "How to Brew." In fact read it at least twice and brew at least two batches before you even expect to understand a lot of this stuff.
3. All of us on this forum who know a little bit are torn between giving tons of advice and urging questioners to search for answers on their own. In the end, search out what you can. It's fun to discover.
4. Why does it taste yeasty? Dunno, probably needs more time.