What is causing my car to leak coolant?

I have a manual 99 camry w/ 135 k. I am having to top off the coolant about every 4-5 days. I haven't noticed any leaks, and exhaust doesn't appear to be smokey.After driving for a long time when i get out of the car, i can sometimes smell burning coolant( I know the smell from when i spilled a little on a hot engine). Any ideas what the problem might be, or how to find out what the problem is. How much to fix, and if it doesn't get fixed right away will it cause damage?