You aren't obligated to one, use several, try them all and find the one you like. Around here it seems Parkingcrew and Bodis are much more popular than Bido.
But, before you have to worry about that you need to find or buy domains that will produce revenue. The vast majority of domains available online will not produce a penny. Since you are just starting, I suggest you spend a few months reading and learning on here before you buy anything.
As far as what you can earn, its unlimited. The better your domains, the more you make. The more domains you have, the more you make. The more you know about parking, the more you make.