Welcome to sailnet.
My guess is you both will not be away from the dock for more than a few days at a time and that there won't be much to use power except some lights
. I'd also guess a small outboard. I think if this is the case one battery should be sufficient. You will need navigation lights
if you plan to sail at night ever and an anchor
light. For the navigation lights
you need a combination red/green bow light and a stern light. If you plan on using the engine at night you will need a steaming light. The red/green is best put on the bow pulpit and a combination is more miserly with power as it uses only one bulb. The white stern light can be mounted on the stern rail if you have one or on the stern as high as possible if not. The steaming light mounts on the mast usually near the spreaders. If you plan to anchor
overnight the best solution is to hang a portable light from the boom over the cockpit.
For the interior you will need a few lights
for cooking, reading, etc. These are best kept to a minimum to conserve power. In areas where you don't need a lot of light on a normal basis you can buy stick on LED lights with their own internal battery - because they are LED the battery lasts ages. They can be purchased at Home Depot or Wal Mart for about $20 for a 3 pack. For the permanent lights small flourescent lights will use less power than incandescent and are a lot less money than LED. Below are several links to on line
places to buy lights.
For a battery box you're probably best to buy a group 27 or 31 deep cycle battery with a plastic box and strap it down under an aft bunk so it can't move. A small fuse panel and a main battery switch should be installed as well. The interior lights will have their own switches so all you really need is a fuse block for them. You will need switches for the running lights, one for sailing (red/green and stern), a second for the steaming light, and a third for the anchor
light. These switches can be fed from the fuse block. Shown below from Defender.
The battery charger
is probably best kept to a portable as this saves the expense of a shorepower inlet and cable. There is a 2/6/10 amp charger shown below from Amazon.
You don't say how tight your budget is nor how you will be using the boat so I've tried to outline a system that will work but keep costs down. I don't know where you are so have suggested online sellers. There are many other options as well but this is about as simple as a real 12 volt system gets and still remains workable. I hope this helps.