I'm going to honest with you fast money is often difficult to come by legitimately and most methods of getting fast cash can be sketchy/illegal (Don't go down that path. I've had friends who've been involved in that stuff and it didn't end up well for them.)
However, here are some legitimate ways you can earn some money:
Get a job- this is the most obvious answer but it is the most reliable way you can earn money. Maybe you won't get fast money, but you get a constant stream of income which is good.
Do yard work for neighbors-walk around the neighborhood ask if your neighbors need their lawn mowed, weeds pulled out, etc.
Resell some of your old stuff/hold a garage sale- Do you have an old iPod?, used in good condition clothes?, a camera you don't really use?, some old video games?, etc. You probably won't get rich, but you can get some extra pocket money.
Do you have any marketable skills to offer- Can you tutor other people in a certain subject?, Can you teach others an instrument?, Do you do photography?, Can you make websites for other people?, etc.