First of all, consult the Writers' and Artists' Yearbook to find out which literary agent is suitable to send your book to. For example, if it's a sci-fi novel, then submit it to an agent who deals with sci-fi.
Next is the enquiry. There are few agents who accept enquiries by e-mail, so phoning or sending a letter are the only other choices. Always check the Writers' and Artists' Yearbook to see whether or not the agency accepts unsolicited manuscripts (manuscripts that haven't been asked for). Many agents don't, as they are swamped with manuscripts every week. If the agent doesn't accept unsolicited manuscripts, then you should certainly send an enquiry letter asking if you can submit a sample of your book.
Always check the submission guidelines for literary agents. This information should be available in the Writers' and Artists' Yearbook or on the agency's website. The majority of agents ask for a synopsis and three sample chapters in the first instance. Some agents may only ask for one chapter.
With all agents, your work should be double spaced on A4 good quality paper, with your name, address, and page numbers on every page. Use sufficient postage for your manuscript (you may have to spend a little extra as a first class stamp won't be enough), and make sure you enclose an sae (self-addressed envelope), with the same amount of postage as before, if you want your manuscript returned to you should it not be suitable for the agent.
Once an agent has your work, leave it at least a month before you contact them asking if they've had a chance to consider your work. Agents receive hundreds of submissions from authors each week, so it may take some time before they can have a look at yours.
If you receive a rejection, then do not pester the agent. Agents do not have the time to enter into correspondance with authors that have been rejected.