Can I make a few suggestions without actually giving anything away? If you know all this already, then ignore it.
1) There are two types of cipher - ones where the same letters are used as are in the message, but juggled around in some kind of system, and ones where each letter is substituted for a different one in the alphabet. Sometimes the substitution is very simple, sometimes the pattern varies.
2) There are 26 letters in the alphabet. The first is A, the 26th is Z.
3) Some letters occur more often than others in any typical bit of text. for instance, E,A,I,T,O,N,S, are a lot more common than Q,X,J,G,B,F. The most common letter is E, so the most common letter in an encrypted message is likely to be an E (but not always!)
4) It may help to write out the alphabet to keep in front of you, either across the page or down one side.. Write it backwards as well, and the numbers 1 to 26. It saves a lot of counting backwards and forwards on your fingers.