Some
marinas want reservations, some want you to call/hail when you are nearing the marina. You should be doing that
research before you make way, so you don't arrive somewhere and have nowhere to go.
ActiveCaptain, Waterway Guide,
cruising guides are the resources for planning.
When you get within range of the marina, hail them. You can call by
phone. When you hail on Channel 16, they will likely direct you to another channel to discuss your needs. Regardless how you contact, tell the marina your
boat type, size, beam,
draft and for how long you'd like berthing. They will respond with more questions, directions or maybe someone will meet you to take your lines. If you don't feel confident about
docking, ask if they have a person to help if they don't offer. If you aren't sure about the approach, ask them. If the marina isn't helpful, choose another.
Always, always have a Plan B. Plenty anchorage options, so you don't have to stay in marinas every time