The brake fluid reservoir is usually under the hood right in front of the steering column. Most cars today have a transparent reservoir so that you can see the level without opening the cover. However most older cars aren’t as easy to see. The brake fluid level will drop slightly as the brake pads wear out. This is a normal condition and you shouldn’t worry about it. If the level drops noticeably over a short period of time or goes down to about two thirds full, have your brakes checked as soon as possible.
NEVER PUT ANYTHING BUT APPROVED BRAKE FLUID IN YOUR BRAKES. ANYTHING ELSE CAN CAUSE SUDDEN BRAKE FAILURE! Keep the reservoir covered except for the amount of time you need to fill it and never leave a can of brake fluid uncovered. Brake fluid must maintain a very high boiling point, and exposure to air will cause the fluid to absorb moisture which will lower that boiling point.