I am guessing you mean the regular games at the casino like Dice and roulette?
1. Nothing. Most of the casinos explain how to play the game. You will find it somewhere around the interface. They are never complicated to play. You place the bet, click a button and two and hope that you get lucky. Most of those games require no skill at all. It all depends on the luck.
2. Both as long as you win. If you are playing casino games, look at the house edge. Lower house edge means better chance of winning.
3. Why not try both? For sports, you might need to know about the team, performance, stats and other stuffs. Well, you don't even need to know those. Just look at the odds and place you bets. Higher payout probably means that team has a high chance of losing (gamble at your own risk. Do your own research!).
These are probably the best advices here. We can give as much advice as possible, but in the end it comes down to personal preferences and what you like the most. Just because one user here is very successful with sports betting doesn't mean that it will be the same for other users. We need to stick to the games that makes us comfortable and where we think that we have chance of winning in the long run. House edge of the casino (or best odds for bets) are important for every gambler. Being critical is also important, don't trust everything you see or read. Making up your own mind is helpful for choosing the best games for you. In case you don't have much money right now, many casinos also offer the opportunity to use playmoney to try out their games for free. It's a good way to get to know all the different types of games with out risking money. And once you become comfortable with the games you can bet your own money. If you are signing up for the first time at a casino many of them have a 100-200% deposit bonus which is a nice boost to your initial bankroll. It would recommend to deposit a larger amount at first so you can take full advantage of such promotions.