First, use a good latex caulk. It is easier to work with by the novice and can be cleaned up after easier. Silicone sticks to everything and is hard to get off. Neither of the caulks last as long around a tub as their life claims and when it comes to removal and clean up, silicone is a pain to remove.
Second, for a straight and clean edge, use the blue painters tape to make straight caulk lines. It doesn't work so well for corners. Pull it up at a right angle within a few minutes depending upon the caulk you use. Too early and the caulk may run and drip where you don't want it and wait too long and the caulk get tacky and it may pull all the caulk with it.
I also think a small tip size is important as well as the size of your finger when wiping.