my battery keeps saying, service battery, please help

Batteries are considered Consumable Products.

Over time it will degrade to a point where it needs to be replaced.

The usage is normal as the capacity will go up and down all the time and never stay always at 100%.

Apple Batteries are rated for 1000 Full Battery Cycles and / or 80% Capacity before needing Evaluation or replacement.

Pay a visit to your local Apple Store's Genius Bar or Apple Authorized Service Provider to have them take a look and provide you with your options. Most likely, your notebook's battery has reached it useful life and needs to be replaced.

You don't fix an alert, it's just a statement telling you your battery needs a "fix"