How good do you have to be to lead climb?
Before lead climbing, you should at least have some experience in climbing and know the basics. Depending on how and where you want to learn lead climbing, you may be required to have some climbing experience. This experience could be a certain amount of time climbing, climbing to a certain grade, or just being familiar with climbing in general, climbing holds, and how to move on the wall. Some places are stricter than others.
If you really want to start off well, it is recommended that you get used to climbing before. This can either be from top rope, some sort of auto-belay device, or bouldering. The more experience you have with climbing, the better and easier it will be to learn lead climbing.
In terms of what level you should be climbing before you can do lead climbing, there is no rule for this. Even if you are a beginner climber, you can lead climb. Still, it is recommended to have experience in climbing before starting with lead climbing.