Pretty much what everyone else has said. If you don't feel comfortable telling someone about your love of tickling, ask yourself why it is? It may be (as you said) because you are shy, but it may also be to do with how intimate you are with the person in question. You'll have to guage how much of it is to do with your shyness on a case-by-case basis.
Having said that, it's okay to step out of your comfort zone once in a while. Most girls I've told about my fetish have said it sounded fun, many of them said they'd be willing to try it, and none of them responded negatively. Compared to some of the things people are into, it's relatively "tame", and in that lies it's strength; it's not nearly so difficult to talk to people about as, for example, scat fetishism.