In the case of a DIY, low-cost green screen setup, a simple overhead ceiling light can work. However, it has to be bright and you have to position yourself so that the light bounces off in front of you and not directly overhead. You can also be creative with your lighting, such as using different types of lamps you already have like a desk, table, or floor lamp.
Whatever your lighting setup is, remember that the best light position is in front of you, not behind, below, or overhead.