Completion Level: Get $10,000 by stealing items.
Reward: CJ becomes fireproof.
In order to run a Burglary mission, you'll first need to complete the Home Invasion mission that Ryder gives you early in the game. After that occurs, you can get in the Boxville trucks in each major city (for example, south of the gym in Los Santos) to begin burglary missions.
After you declare your enter to buglarize, you have to find a house with something worth stealing. These are indicated by yellow arrows in front of their doors, but these arrows can be difficult to spot from the street, so you may have to get up on the sidewalk to see them. When you locate a likely target, leave the truck and head inside.
While you have your burglar mask on, you can creep around and not make too much noise; don't forget to crouch for even greater stealth capabilities. Inside a house, you should find numerous items of value; pick them up with your triangle key and bring them back to the truck. After you fill up the truck or empty out the house, return to the lockup indicated on your map to drop off the goods. You'll earn twenty bucks for each item you stole, along with a bonus for the total number of items that you returned.
Of course, this wouldn't be much fun if there wasn't any risk to it. If you make too much noise, or are seen by one of the house's occupants, you'll have ten seconds to get out of the house, or you'll earn a wanted level.
Regardless of whether or not you're in the process of burglarizing someone, the mission will end at 6:00; any items in your truck that you haven't delivered to the lockup won't generate any cash. Since you need $10,000 from the burglary missions to achieve 100% game completion, your best bet is to go for big bonuses by stealing items from multiple houses up until 4:30 or so, and then delivering them all at once.
Completion Level: 12
Reward: CJ becomes fireproof.
Firefighter missions are almost as difficult as Ambulance missions, although for different reasons. With Ambulance missions, you have to be very careful not to damage your vehicle, while with firefighting missions, the firetruck will be fairly durable (although you still can't leave it or take it into the Pay N Spray, so be careful), but you'll find yourself on the razor's edge timewise.
When you enter a firetruck and click R3 to begin the missions, you'll be pointed to a burning car; head towards it and use your L1 button and the right analog stick to douse the car with water, putting it out. Sounds easy enough, but for each subsequent mission, you'll have to douse both a car and its passengers, and said passengers will be running for their lives. It's the running part of it that makes these missions difficult; people on foot can run into places that your firetruck simply isn't maneuverable enough to reach. You'll need to peg them with water before they get too far, then, or you'll waste most of your time tracking them down.
Eventually, you'll have to deal with multiple cars, each with multiple occupants, for ultra-crazy hardness! When you have multiple cars, try to spray them one at a time, dealing with all of the occupants of a car before moving on to the next car in the line.
Completion Level: 8
Reward: Obtain RS Haul as an asset.
There are eight trucking missions in all; these are discrete, designed missions, not random affairs like the emergency vehicle missions. For these, you'll either have to deliver a load to an area within a specific amount of time (being late reduces your reward); deliver a fragile load (taking damage reduces your reward); or deliver stolen goods (which will bring along with it a constant three-star wanted level).
Of these, the most difficult is the last sort. You don't take a lot of damage in your semi, but you will take damage, and if you ever have to stop for any reason, you can expect to get blown away by the cops' bullets. If you can find a route to your destination that doesn't involve tunnels (which will often be too small for your semi and load), you should be fine.