Matt -
This is not a "get rich quick" thing. Most companies will pay you about six weeks after you complete the work. There are a handful that will pay you more quickly, but this often means lower compensation. There are a few things that will pay you almost immediately. I don't use them, so perhaps somebody else will tell you about them.
Overall, you are starting a business and becoming successful will involve a slow start and a large investment of time. As you just get started, you might struggle to consistently earn minimum wage once you factor in driving times. However, once you have established yourself, you can do much better.
If you want to establish yourself as quickly as possible:
1. Read the "New Shoppers" area on this forum. It will really orient you to the world of mystery shopping.
2. Keep up with this forum. There are folks who have been doing this a long time who have made a decent living doing it. A few of them willingly share their knowledge here.
3. Sign up with as many companies as possible. This will take a lot of time, but it is well-worth the investment.
4. If you decide that this is for you, invest in video equipment. This is where the best money tends to be.
5. Always treat schedulers and company owners respectfully - even when you think they are fools. They control your work.
6. Never flake on assignments and always do very high quality work.
7. Figure out your niche. What types of shops do you prefer? What are you comfortable doing? Will you travel? Etc.
Hard work builds character and homework is good for your soul.