No, you don't want to just caulk over the old caulk. Scrape out the old caulk (there is a little curved tool you can buy for this at any hardware store, or you can use the edge of a plastic puttyknife). Clean the surface as well as you can,using a sponge, soft scrub brush or old toothbrush. Once it is clean and dry, re-caulk using a mildew-resistant bathroom caulk. Wear gloves--it's sticky and can be a pin to get off your hands. You might want to practice laying down a line of caulk (called a bead) on a piece of paper first--if you are good at writing with cake frosting, you will find this easy. If, like me, you got C's in penmanship as a kid, this is less easy, but utterly doable. The trick is to not to use too much at a time, and to lay down a consistently thick amount. You then want to go back over it with your finger, smoothing it to make sure it fills the line between the tub and the wall. Have a roll of paper towels handy--messes can be cleaned up with a damp towel. If you mess up, scrape it out and try again. Also -you don't have to use a caulk gun --bathroom caulk also comes in tubes much like toothpaste tubes, and one tube is sufficient to do one or two tubs. Good luck!