If you've got the right equipment, the next step is to check the submission requirements for the various libraries. These will vary from place to place. Some companies allow you to upload low-compression JPEG files directly to the site, while others require you to send in large uncompressed TIFF files on a CD or DVD. All of them will want to check that your work is of sufficient quality before they accept it, so you should put together a small portfolio of your best shots. These should be as varied as possible, and include generic product shots, landscapes, portraits, action and lifestyle photos.
At this point it is important to note that all stock photo libraries will require that you have full ownership of the photos you submit, and that you have permission to use those images commercially. It's important to make sure that your photos don't include anything that is already copyrighted, and this includes product logos, brand names, and even some shapes. Sometimes these can be quite surprising. I had a photo of London including the new City Hall rejected by a stock library because the building itself is a copyrighted design.
For photos of people, it is important to get formal permission from the subject to use the image commercially. This is usually via a model release form, which the model must sign to give you formal legal permission to use the photo. Stock libraries will usually not accept photos of people unless they are accompanied by a signed model release form. Most of the libraries have model release forms available for download on their websites. Download one, print off a dozen or so copies and keep them in your camera bag.
Once your work has been accepted, most of the libraries will give you the option to select what type of licence you wish to apply to it. What this means is whether the photo will be sold to a single user for a one-time use, usually for a single large fee, or whether it will be made available royalty-free, so that it may be used by several different customers over many sales, but usually for a smaller fee each time. Read the details of each company's licence terms carefully, because this will affect how your photos sell, and also how much money you make from them. The libraries themselves make their money by charging a commission each time one of your photos is sold, so be sure to check how much of a cut they are taking. The larger, more professional companies have very reasonable commission rates, which is why they are so popular.