In order to get to the Proprietor at the stadium and theater, due to the fact that they are rabbit holes, you will need to click the blue icon in map mode to call them out. You can also click your Sim to call the proprietor on other lots.
There is a competition in town called SimFest that will pop up from time to time. You can talk to the proprietor while on the lot to get the option to join. If you win the competition, you will earn a prize based on the quality of the venue (and thus the competition). These don't pay nearly as much as a solo gig, but they do give you tropies and some quick cash. Use the crowd after (and maybe during) the show to get some tips.
You will fail at tricks, and get booed a lot at your first few Magic Shows. People will throw bad things at you like fruit, but may later throw flowers once your Sim is nailing all the illusions. Sell any food before it spoils, and note you get a little profit from failure as you clean your inventory.
Your stage presentation is important to the rating of your show. Using the venue's default is in general a bad idea unless you're broke. Investments into props are permanent, as they are stored in your Family Inventory. So, anything you buy while setting up your stage is ready for use later. You can save one stage setup and reload it each time. It gives you the option to modify it, and for some annoying asks you if you want to overwrite the saved copy even if you did not modify it. When you press F1 to enter Live Mode after setting up the stage, the show begins. Note that you can start a show early and it seems to help your performance rating. That improves the chance of a steady gig but doesn't impact money. In fact, you can still make the same money but get a lower rating and reduced chance of a steady gig even if you're an hour late.
The amount of money you earn for a show, good or bad is fixed. The earnings are based on the quality of the venue and your current level. At base, doing the Big Venues will give you $18,000 at level 10, Private coming in at $11,000. Those two types are more than enough to survive and grow in wealth in The Sims 3. If you continue past level 10 and keep gaining levels in the Magician Profession, you will earn more and more for each performance.