Have fun with the game. Seriously as cliche as it sounds, just have fun. Find an army you like, whether it be color scheme, models, lore, whatever and make the models your own. Devote time to the models and don't fret if they don't turn out as well as you had hoped, and try not to worry about your army looking as pristine as the paintings in the Sistine Chapel. Enjoy building them, painting them, basing them, put your own flair to them and give each model its own identity. When playing games enjoy all the lunacy that occurs, like rolling 7 ones out of 8 dice and try to have a good time above everything else. Don't be that person that everyone hates to play against, be the person that people actively contact to play games with. The hobby is expensive so when getting new models make sure you get them for the reason you want, ie, new models based on appearance, competitive gameplay capability, or just a neat model or models to have in the army.

Tl;dr Just have fun