This is just a summary of the main concepts. There are entire books (Larry's book) on the topic and it is hard to summarize in a brief article. Here are a few guidelines.
The 2 most desirable leads are:
1) Top of a sequence, preferably 3 in a row such as KQJ or QJ10.
2) Partner's suit. Lead the card you would lead even if he hadn't shown the suit (so LO