2003 Nissan Sentra - Need help starting

my car is not starting even after jump starting, need a mechanic to help with replacing the car battery.

You don’t need a mechanic, most places that sell batteries will install them.

Why do you think you need a battery? Just because it isn’t starting? Does it crank and not start? Or does it just click?

If you do need a battery, AAA will deliver and install one for you (if you are a member). And they will also diagnose a starting problem for you at no added cost.