Start by collecting what you like...

You have to live with the art on your walls so only collect paintings, prints, and sculpture that you respond to in some way. If you don't like it, why hang it in the first place? It has to move you or challenge you in some way. You can respond to the subject matter (a landscape, an abstract arrangement) or the artist's technique. Or it could be the color palette that evokes an emotional response. Whatever it is that attracts you to a particular piece, go with it. Some people like to collect a particular artistic style like Impressionism or Abstract Expressionism, or art from a particular time period. Some are more eclectic and will buy whatever strikes their fancy. There is no right or wrong so trust your instincts, learn all you can about the art that interests you, and then buy what you like.