I’m beginning a job with Doordash and I need to know how to file taxes as an independent contractor and how to do the mileage deduction. I also have a regular second job

Document EVERYTHING. Particularly vehicle mileage. You need to keep a log of "ALL" miles driven for both personal and business. You will also need to document your starting mileage on Jan 1st or the first day your business opened, and your ending mileage on Dec 31 of the tax year.

With vehicle expenses you have the choice of actual expenses, or the per-mile deduction. But when using a vehicle for less than 100% business use, it can be difficult if not impossible, to prove actual expenses. Besides, the per-mile deduction is almost always better for a business that involves a lot of vehicle use.

The above only applies to a vehicle you actually own. If you do not own the vehicle you use for this business, then you must claim actual expenses and be able to prove you paid those expenses if audited.