You may be thinking asking for money that is not a hard problem for me to do. When we as designers no matter what profession people will think they can get us to do whatever for free. Honestly if you are upfront with them about payment you can find out who is serious and who is not.
Because you should be feeling as if your work is good enough to have people pay for it. This is one of the things that has to be learned through trial and error. The other thing is to be able to sell your work to a client. If you have a client and they say they like your work and are trying to get you to work for free be honest with them but not an asshole about it. You are a professional so start acting like one.
Another thing is asking for more for the amount of work you are doing. If you are low balling yourself and not being able to continuously make a living and even more then you have to start knowing how to ask for more money for your time. Of course over time your skill set will improve. Then you increase your prices a bit. So stop being afraid to ask for money when your doing work with a client or whomever.