I'm prior military and a mother. Idk your due date but it sounds like you will have to delay going to basic training. There is also more you will need to do as in custody of your child and care while your gone if you still want to go. If you are having financial difficulty I recommend going to WIC they can give you some aid and point you in the right direction for anything else you need.
This is actually my second pregnancy I'm also 21. You will do fine I was terrified I wouldn't know what to do but you just somehow do. As for the basic training I'm not to sure I would imagine that if you told them your circumstances they may delay it or not allow it.