You should be propertly prepared.
There's really more to painting than buying a paintbrush and a ladder.

Here are the basics required:

1. Knowledge of painting.
    If your only experience is painting your own home, that may not be enough.

2. Tools and equipment.
    There are some basic tools and equipment every painter should have.

3. Be able to prepare an estimate.
    This part is important.
    If your price is too high and you won't land jobs.
    If your price is too low, you may be working for minimum wage or less.

4. Know how to present yourself to the customer
    and how to "win friends" with your customers.

5. Marketing: You're going to have to get leads--prospective customers.
    There are several ways to do this.