You need an AC panel and one good source is Blue Seas. I'm not going to give you a link, I'm going to tell you what you need to do. The battery charger needs to be large enough to accommodate your batteries. So, if one of them is a deep cycle battery, you need at least a 10 amp charger. The bigger the better (charges quicker), but bigger means more money, so shop for the biggest you can afford and make sure it is auto, meaning it will regulate the condition of the battery charge and slow the charge down instead of just over-charging it. You have two types of chargers. One has a 3-prong plug in to go into an inlet and another is hard wired into the AC system. I like the latter, but the choice is yours as they both work fine. If you go with the plug, then you need to wire in an outlet from your AC panel. (A duplex outlet located near the charger so the cord can reach). If you hard wire the charger in, you need to get a charger with enough banks to accommodate the number of batteries you have. Pick out your AC panel to accommodate all the things you intend to run from it (you need a breaker for each load). The breakers must be marine types (don't use household Square D boxes, for example and use stranded wire 12-ga/3 conductor (white, black and green) and not solid core like they use in houses). The main breaker is a double breaker meaning it breaks both the hot and neutral wires. This is where the wires from your inlet (shorepower cord) come from. Then you will have another breaker for your outlets (maybe two for port and starboard if you have a bunch) and another one for your battery charger so you can turn it off from inside the boat. If you are ever going to install refrigeration, an inverter, or any other thing that uses AC, then you want a breaker for each purpose or load. The breaker you use is one that is the correct capacity for whatever you are installing. The sheet that comes with the unit usually tells you this info.
So, if the only thing you are going to have is your shorepower and a battery charger, you need the one panel with the double breaker and a spot to add a breaker for your battery charger. The panel should come with labels for you to affix. If you are going to run a battery charger and some outlets, then you would have the AC main breaker and two spots to add a breaker for the charger and another for the outlets. From the charger, you would wire to the breaker panel. The green goes on the green screw, the white goes on the silver screw and the black goes on the gold screw (this should be detailed in the instructions with the panel). Of course the shore power is not plugged into the inlet on the boat while you work. All juice should be off.
This should get you going. I can't think of anything else at the moment. Ask if you have questions.
