there will be high priority cars he wants which will pop up green on your map. That car will give you the biggest payout. The other ones he send you in a text message, you just have to know what they look like and bring it to him. sometimes i just happen to randomly steal the car and it says Simeon wants it.
There are two types of missions with Simeon.
In one, he sends you a request for a specific car or two. Those will appear on the map.
In the other, you get a list of cars that he wants. It will just be a list with very little additional text. The catch here is that you can only get one of them...once you bring one car to him at the docks, the list disappears. He'll send you another one with a different list eventually.
The cars he wants tend to be very high end, but the most I've received for one is $6,800, which makes this less of a good deal than jacking a Baller and dropping it at the nearest Los Santos Customs. Still, if you're up for it, it's not a bad way to grab some cash.
You can almost always find the cars that he wants around Michael's house, or in the driveways on the hill behind North Vinewood. You must respray the car before you can deliver it to Simeon. The Classic paint options are the cheapest way to go with this. You can get a couple of different shades of black for less than $900, plus you'll want to repair the car to get the full value from Simeon.
By the time it's done, you'll wind up pulling $5,000 to $5,900 from one of these deliveries.
he's talking when you receive the text message...
here's a picture of it sorry for the french version:
This mean that you need to pick either one of those cars, as soon as you will enter one on the list, you'll get the cops chasing you and a message stating to loose the tail.
once tail lost you'll need to cover the paint swap at lost santos customs then deliver the car to Simeon at the docks.