With some cars, you can use automatic transmission fluid, but others require special power steering fluid. Look it up. With a plastic master cylinder reservoir, you can see the brake fluid level from the outside. You'll have to remove the cover of a metal reservoir. Keep them nearly full.
Many reservoirs have two separate chambers.
The fluid level in the one that goes to the front disc brakes will fall as the linings wear out. This is normal. Brake fluid has an affinity for moisture, so make sure the reservoir lid is on tightly, and keep the can's cap snug. Brake fluid is a strong paint remover, so don't drip any on the fender. The windshield washer fluid level is visible through the side of the reservoir.