how to clean inside of the macbook?

Assuming this is a unibody model, you'd have to remove the bottom cover and then being very careful, blow air into the fan blades and anywhere else you see dust bunnies. But just remember the battery is connected, so there's live current running through the system. Be very careful about touching anything. You'll need a Phillips #00 screwdriver to get the bottom cover off/on. I have managed it with a small Phillips #2, but you run a greater risk of stripping the screws if you do that.

Using compressed air to remove dust is one of the worse things you could conceive of doing to your computer. Compressed air will force dust into crevices, and make i impossible to use. It can cause a fan to spin at abnormally high speeds, forcing the fan out of balance and damaging the fan bearings. Worse, when compressed air is released, the temperature of the air stream is extremely low (physics, you know), and can cause condensation, damaging the electronics.

Use a vacuum cleaner judiciously.

"I suppose."

Either it does, or doesn't work, I suppose. Which is it?
Get a stopper, rubber or cork, that fits into the vacuum cleaner hose. I suppose you should also get a drinking straw, and drill a hole into the stopper. I suppose putting the straw into the newly drilled hole is the next thing to do. When you turn on the vacuum cleaner, the suction at the straw hole will be sufficient to remove all dust and debris. I suppose.

DON'T use a vacuum cleaner. Canned air is actually probably the best agent for cleaning your computer, despite what some people are saying. The key is that with laptops in general (macbook forsure) you don't clean it through pointing it in the only hole. To clean it properly, that means taking off the bottom so the compressed air has a clear path to get the dust out of the case. To do a better job, removing of fans and cleaning is pretty easy (as it tends to gunk on the blades). If you attempt to clean only through the vent, it will help nothing.

If you are scared of working with computer materials, have a computer store clean it.

"Canned air is actually probably the best agent for cleaning your computer, despite what some people are saying."

You can use my name - it won't offend me. While you're in college, kid, you ought to learn the difference between doing something right and doing something wrong. Your sage advice is based on what? How many years have you spent designing portable computers, and developing methods to repair them properly? Ah, zero.

Well, I worked multiple years for Toronto Hydro designing, repair, cleaning the server boxes they deployed for monitoring and control of the power output. I now work for the government in computer support, I hold A+ Certification and a CST diploma, and I run my own computer company (small scale hobby).

What are your reasons for not suggesting canned air, and suggesting a vacuum cleaner instead? A vacuum is a lot more capable of passing static electricity to the board (frying it). Do a little research yourself please:
( http://www.computerhope.com/cleaning.htm)