To answer your questions, it would be better to know which service you are looking into as the regulations differ from service to service. If you are joining the Army, then you cannot regain custody of your child until after your first enlistment. In fact this is pretty much the rule of thumb for all the services now. Again, depending on the service and how the custody agreement is worded will determine whether or not you will be able to receive any financial support. In most cases you cannot. If the person you are giving custody to is in the service, then there are regs that they will have to follow if they are allowed to even gain custody at all.
What lies behind us and what lies before us are tiny matters compared to what lies within us.