How do I stop Facebook from randomly posting ads/ stuff on my friends' walls?

[Samantha]

I check my friends' walls and it says that I posted things like "Check this out !" with a link. I don't know how to stop it though.. help me please!

[starz530]

for starters, change your password. Then stop clicking on those sorts of links on other people's pages

[Tasha]

Your account was phished. Change your password and don't click on any similar links in the future

[Ascendancy]

The other two are right about changing your password. However, whenever you take a quiz or use any application, it asks you "Post this randomly to your Friend's wall" or something of this sort.
Just deny and nothing will be posted.

Hope I helped!