Although you aren't initially able to do much more than bet chump change at most gambling establishments, there are a variety of games available at the casinos in Las Venturas. In order to unlock higher-limit gambling, you'll have to build up your gambling skill, which you can check with in the Stats - Player screen in the pause menu. In order to do so, you'll have to partake in any kind of gambling that's available to you.
When you first arrive in Las Venturas, you're unlikely to have much of a bankroll, so gambling can be a great way to get mucho dinero in short order. There are plenty of games in the casinos and some, such as pool, available elsewhere, but we'd recommend two of them as being especially fun and efficient.
First up is Video Poker. You can find banks of these machines in Wu Zi's casino in Las Venturas, just inside the entrance, so if you're interested in increasing your bankroll, save your game inside, then head in and start pumping quarters. There are machines for every betting range from one smacker to 10,000 bucks a hand, and since you can enter as many as five coins at a time, you can get up to a 50,000 dollar wager for each hand on a 10,000 dollar machine. If you don't have a huge amount of money to work with, though, try to place a bet that's around one-fifth of your total bankroll, if only so you don't have to reload after each hand.
Your three most basic plays here are:
1: Hold any pairs you get. If you have a pair and flush the rest of your hand, you'll have a 13% chance of drawing three of a kind, which will triple your bet. If you have three of a kind in your opening hand and hold onto all of them, you have around a 4.5% chance of getting four of a kind, which is the highest payout you're likely to ever see. (If you're betting 50k a hand, you'll get 1.25 million back on four of a kind.)
2: Hold any four cards of matched suit and hope for a flush. If you hold four cards of a suit, you have an almost 20% chance of getting a flush. Thus, you should go for this pretty much always, even if you're holding a pair. (If you're holding a pair of jacks or better, you'll have to weigh the 20% chance of sextupling your bet vs. 13% chance of tripling your bet vs. the 100% chance of making your money back.)
3: If you have an open-ended straight draw, meaning that you possess four cards in numerical order (e.g. a 6, 7, 8, and 9), then you have a 17% chance of drawing into a straight with your last card. Although the chances of this occurring are lower than drawing into a flush, straights are much more common than flushes (because the cards involved can be from any suit), so the payout isn't as great, giving you back only four times your initial bet. Still, if you have to flush a pair, then you'll probably want to do so, with the same risks as a flush applying.
These are the most common situations, but you'll also come across opening hands with no-brainer situations. E.g., if you get three of a kind in the opening hand, you hold them, or if you draw into a first-hand full house, you obviously hold that. You can play with greater risk if you have plenty of money to spend, but while you're building up your Gambling skill, play it safe and make your money. Depending on the stakes you're laying down, you'll probably want to save your game after increasing your bankroll by 20% or so; you don't need to save it every time you increase your cash. (Although you can feel free to do so, of course.)
After you get to a point where they'll let you in at the Roulette tables, hit the save spot, then bet all of your money on either red or black. If you lose, reload; if you win, save again and try to double up again. The max bet at these tables is a million dollars; when you're allowed into those, you can gain tens of millions of dollars within a half hour or so of betting and reloading.
Note that your gambling abilities are affected by your Luck stat, available in the same menu as the Gambling skill, albeit further down. This is a numerical stat, without any kind of objective reference to compare it to, but apparently higher is better! If you want to get a high Luck stat, you can collect Horseshoes from around Las Venturas; getting all of them will give you a big boost to Luck.
There are two triathlons in the game, which will require CJ to be in top physical shape, as they consist of biking, sprinting, and swimming portions. These aren't required to get a 100% complete game, but they're fun to try! These are only available on Saturday and Sunday, though, so they're easy to miss. If you want to check them out, head to the beach near the lighthouse in Los Santos. There's another triathlon north of Los Santos, on the beach to the southwest of the town of Palomino Creek.