A cryptogram consists of a piece of text, typically a quotation, where every letter in the original text has been replaced by another. The same replacement letter is substituted for each occurrence of the original letter. The solver must decipher the code to discover the words in the original text.
Without wishing to spoil all the fun (look away now if you don't want to know) there are several pieces of sound advice that can help with the solving of monoalphabetic substitution ciphers. Some of these are given, below.
Apostrophes are helpful. If one comes before the last letter of a word, that letter is probably an S or a T. Single-letter words will almost certainly be an I or an A.
Watch out for recurring three-letter words – they could be AND or THE (and if you find a four-letter word starting with the same three letters, you might have stumbled on a YOU/YOUR or THE/THEY combination).
Double letters are a good indication – at the end of a word, for instance, they are probably a pair of Ss, with an E preceding. And at the end of a three-letter word, they might suggest either ALL or TOO (although beware of EGG, OFF, EBB, ILL etc!).
A word of five or more letters with G as the final letter may end ING; an I has the third-to-last letter could indicate an ING or a TION ending.
H is almost invariably preceded by C, S, T or W.
The solver shouldn't assume that the most frequently repeated letter represents an E, as it is quite possible to have a sentence with hardly any Es in it.