My 10 year old wants to use computer- which parental controls should I use? Free is better...

Great question! I bumped you!

What kind of computer - different types have different things built in already. The most basic is setting up an acct for him separate from the admin and changing what permissions it has (make it not allowed to download anything, to start).

I love K9. It's free and works really well.

I wouldn't allow free access to Youtube at all. I don't know of any kid version, and sometimes things that look like "normal" "okay" videos turn out to be pretty bad, even for adults.

This is how you enable the parental controls on Windows 7:

http://windows.microsoft.com/en-US/windows7/products/features/parental-controls

Ours are mostly focused on what he is allowed to download or install (NOTHING) 

He has his own computer, but if he was on ours, we'd also use the program lockout that prevents him from using any games we don't want him playing

Make sure you have your computer set up before you worry about adding internet.  No parental control filter is perfect, and it only takes clicking one deceptive banner ad to kill the whole machine.

Parental controls are a great tool and you have already gotten some great advice on those I just wanted to add that nothing can substitute for good old fashion parental supervision. Make sure the computer is being used in a public area of the house and that your child is clear on the do's and don't of internet use as far as clicking on banners, ads or unfamiliar links and downloading times without expressed permission and of course no sharing of personal information anywhere. At our house we have a list of websites they are allowed to visit and anything not on that list they have to get permission. sometimes the answer is 'no -not until I can check it out myself. Ask me tomorrow'

Youtube is direct supervision only in our home.