How do you practice at video games

"Practicing" videogames isn't possible to answer. You don't practice videogames, you simply play them for fun. If your question is how do you play a videogame competitively? Then that itself, is a really broad question. No one videogame is different. Its like asking the Man Vs. Food guy how do I practice at eating? Each food item has their own way to approach it. Or asking a mechanic how they fix a car? Every car is different. Just like every videogame is different.

FPS, or first person shooter requires a quick reflex and quick thinking. That's to become a general player in FPS's. If you want to become better at a specfic game, you must play it over and over and over again. Play against computer controlled AI players first, then play against humans. Test out every weapon until you find the one that suits you, and make sure you find a comfortable control style in the options menu. Memorize maps by playing them over and over again. And make sure to watch others play the game as well, just watching anyone do anything, instantly makes you a little bit better than you were before you watched them play.

Try just playing whatever game your playing over and over. Don't force it though. I don't know anyone who says they enjoy EVERYTHING they play. Sometimes, you just have to find that game you love, and you'll find that you'll master it within no time.