For the most part, you can use either enquire or inquire and not make a mistake. These two words, however, have traditionally been used in a slightly different way, at least in the UK, which kind of makes them a pair of commonly confused words. Here’s a tip: Want to make sure you...
The other answer is correct too, in that inquire is regularly used in both situations in america, whereas Britain prefersenquire. Unless you are writing high level business documents or legal documents I wouldn't worry about it too much :) ...