Steps:
1.	Determine whether your car has a hydraulic clutch. (Image a.)
2.	Turn the engine off and open the hood.
3.	Look for a small plastic container about 1 inch in diameter located close to the back of the engine, usually near the brake fluid reservoir. It looks a lot like the brake fluid reservoir but it's smaller. Imagine that the clutch pedal went straight through into the engine compartment: this is where you'll find the clutch master cylinder and clutch reservoir.
4.	Check the fluid level. The reservoir is usually clear with a small round rubber cap on the top; it should be filled to the top. (Image b.)
5.	Add brake fluid if it's low. (Image c.)
6.	Replace the cap.
click photos to enlarge
	a. b. c.
	Tips:
	The hydraulic clutch uses brake fluid. There's no such thing as "clutch fluid" - you'll get laughed at if you go into an auto parts shop asking for it.
	Cars with a clutch cable do not use any fluid.
	A leak can often be seen at the reservoir/clutch master cylinder, at the other end of the clutch hydraulic line at the clutch slave cylinder, or inside the car behind the clutch pedal.
	Warnings:
	Take care not to spill brake fluid on yourself or the car's paint - it's highly corrosive. Wash your hands and wipe any spills with a rag.
	If the clutch reservoir is consistently low you probably have a leak. The reservoir is very small so even a little leak can empty it out quickly. Without fluid, your clutch pedal is useless so you won't be able to shift, or to drive. See your mechanic if you suspect a leak.