You are almost always better of with an attorney assist you. Although "they should go pretty easy" you are 20 years old and hopefully have a long future ahead of you. A theft conviction will affect that future. It is a crime of dishonesty and something potential employers frown upon. A lawyer might be able to assist you in avoiding a conviction.
You need an DUI attorney to review all the evidence against you including the officers qualifications to truly assess the chances of beating your DUI. Regardless of what your odds are for beating the DUI case, hiring a DUI lawyer will be better than anything else you can do.
The mere fact you got arrested 9 years ago for DUI does not mean you were convicted. We need to know if the arrest from 9 years ago resulted in administrative action or a conviction before determining if this is a 2 or 3rd offense unless CA counts arrest as priors for enhancement.