I actually disagree with telling your mom right now. If she doesn't want you to be with someone, she is going to be overprotective. Sometimes, the best way for you is to learn on your own. Even if it's the hard way. All she going to do is say you're not ready instead of giving you tips to improve yourself so that you are ready. I think the best you can do is do the best you can in building up your school credentials, driving ability if applicable, job prospects, and/or social circles. Do these things on your own. Eventually, plan to move out to an apartment if you can. Instead of asking her for her dating approval, just show her. Just start dating on your own, and also start doing all these other things to make yourself more independent. More independence makes you more attractive to more people in case the one you are with does not work out.
You don't need to tell your mom anything until you and the other person are getting real serious since she is not supportive. Be prepared that she still won't approve, be prepared to stand your ground politely but firmly, and look for alternative places to live as a backup just in case if you can. Leave the place, even for just a day at a time, if you must, but always give her a chance to offer her opinion and ask her to give specific examples of what you can improve on so that you can choose to be your own person that much more. If she's working with you, work with her. If she doesn't understand how to, it may be best to leave for a day and then come back the next day. Leaving is a way to show her your stance. Eventually, you can't do this forever. As long as she wiling to let you coexist, you don't have to permanently leave. All things to consider.