This process will entail some paperwork. First off, you'll have an agreement between you and the agency. The agency will provide this agreement, and you will generally agree online without needing to sign an actual piece of paper. This agreement will spell out the rights and obligations each party has, and you should read it carefully before you agree. In particular, you will want to know how they handle exclusivity and payments. Exclusivity may prevent you from sending your photo to other agencies, or even signing on with another agency, so read carefully.
One important piece of paperwork is called the Model Release, and all agencies should be able to supply you with a generic form. You will want to print out a supply and keep them in your camera bag. You will need to have one of these forms signed by every recognizable person in your photos. There are exceptions to this rule, primarily for photos that will be used for news or educational purposes, but you will need to investigate this further with your chosen agency. There may also be a Property Release form for use with certain properties that require releases. I have no idea which properties may require releases, but you should assume that any photos taken on private property may need a signed release.
Payments will vary by agency. Some pay more for higher resolution photos, or videos. Some pay more based on how many you've previously sold. Some may pay via PayPal, while others may cut you a check when you pass a threshold amount. Make sure you know the details before agreeing to anything. Rates will, of course, vary for any number of reasons, but you could expect to make maybe 20 cents every time your photo is purchased. It's not much, but a popular photo will continue to sell every month, so it could add up.