Mainly practice. Once you've got a sense of your play style you can find openings that fit it and study the books on them, and you should do the endgame drills at some point (at least you should know how to win vs a lone king with a king and rook (easy), king and two bishops (tricky), king and pawn (position-dependent, not always possible) - I've literally never needed to do king, bishop and knight). But the main thing is to just play a lot of games.

There's some great chess people on youtube - I'm a fan of John Bartholomew. He has playlists called "Chess Fundamentals" and "Climbing the Rating Ladder" that are geared towards beginners who know the basics but want to start improving their play. Practicing "tactics" on sites like chess.com, lichess, or chesstempo is another good way to improve.

Learn the moves for the other pieces. :P

This was here a little while ago and it is great: http://www.chesstactics.org

I was acting like I was one who knew the rules but was "just making sure I remember them correctly"