Marinating is a way of flavoring meat as well as helping it stay moist while cooking. Basically you want to submerge the meat in the marinade for 30 minutes or longer. This allows the marinade to soak into the meat adding flavor and moisture. To help the marinade penetrate deeper into the meat, so it's not just on the surface, it's helpful to stab the meat a few times with a fork before placing in the marinade. I find that lightly beating the meat, as though you were massaging it, before marinating also helps. You can marinate in a bowl, add just enough marinade to cover the meat, but I prefer using a Ziploc bag, squeezing as much air out as possible so it's just the meat and the marinade in the bag.
While your grocery store probably has a section dedicated to just marinades, don't limit yourself to just that. Pretty much any liquid can be used as a marinade. For example, I routinely marinate chicken breasts in Briannas Lemon Tarragon or Santa Fe salad dressings.