You can attempt the Valet Parking missions at the Vank Hoff hotel in San Fierro, but you'll need the Valet uniform to do so. You'll obtain the uniform as a part of your core missions, so you can't miss it.
When you start the Valet missions, you'll be given two minutes to take a set number of cars from the car port to the parking garage beneath the hotel. You start off with a three car requirement, and will eventually be asked to park up to seven cars in the same two minutes, but this shouldn't be difficult to accomplish, since you gain time on the clock when you successfully park a car. If you manage to get the car to its space without damaging it, you'll automatically gain back 45 seconds; you can earn another 15 seconds if you align the car properly within the lines. The former bonus is much more important, obviously, so take care not to hit anything on your way down into the garage.
One thing to note is that there are other valets competing for your cars. You can kill them if you wish, but you'll incur a twenty second penalty if you do so. In general, they're not worth bothering with, as they'll only grab cars when you're in the garage; when you're at the carport, you're pretty much guaranteed to get the cars as they roll up.
When you do grab a car, drive it through the small gap in the planters on the edge of the hotel, then make a sharp right and head down into the garage. After a few tries, you should be able to do this without incurring any damage. Getting back up to the carport is a lot easier if you have a lot of stamina for sprinting; if you don't, grab another car from the garage and use it to speed up the ramp.
Completion Level: 50
Reward: Nitro on all Taxis
The taxi missions in GTA: San Andreas are a great way to make some early money while you're tooling around in Los Santos, and are also required for 100% completion of the game. They're not horrendously difficult to pull off, but getting up to level 50 will take quite a bit of time, as you have to deliver 50 fares in a row, without getting out of a taxi.
If you've played Taxi missions in either of the previous GTA games, you should know the basics of this. First off, find a Taxi or a Cabbie, jack it, then hit the R3 button to start the game. If you're in a city, you should spot a fare immediately; they'll appear on your radar with blue icons, and in the game world with blue arrows over their head. If you drive up next to one and stop, they'll get in and give you their destination. The first few fares will be short drives, but as you get farther up the ladder, you're going to have some lengthy drives ahead of you.
Of course, you'll have to complete each fare within a certain time frame in order to not fail the mission. You have a master clock for the taxi missions in general which is added to each time you pick up a fare; the farther away the destination is, the more time you'll be given to reach it. Each individual fare will have his or her own small tip meter, as well; the longer you take to get them to the destination, or the more damage your cab takes en route, the less money you'll get from the fare. In the end, though, the money from fares won't be significant next to the amount of money you'll get from the mission itself; you gain a bonus of $100 dollars for delivering two passengers in a row without ending the mission, and this bonus will snowball into a huge bonus if you can string together a few dozen fares in a row.
The trickiest things about the cab missions are getting to your destination quickly and keeping your cab in good condition. Getting places in a short amount of time is a direct function of your familiarity with the game world, but then, doing taxi missions will increase this familiarity, so there's something of a positive feedback loop there; the more missions you do, the better you'll be at them. Keeping your car undamaged is a bit more difficult, as you'll obviously hit a few cars and obstacles here and there. You can take the taxi to a Pay N Spray, if you must, and you probably should after every few fares, especially if you spot on while you're driving a fare along. You'll probably sacrifice all of your tip money if you get sprayed during a fare, but that's inconsequential when you compare it to the fact that you'll be able to keep driving longer.
One thing to note is that San Fierro, with its many hills, is a difficult place to do taxi missions in. If you're planning to go to Level 50 for your 100% complete game, you should start in either Los Santos or Las Venturas, preferably the latter.