Hi,
First off you'll need to register with environmental health as a food producer and make sure your butcher is registered/inspected, you'll need to have thought about how you'll transport/store the meat to keep it at the right temperatures for selling and what you'll do with any that's left afterwards. And you'll need insurance to cover your 'product'.
We time it so our pig goes off the same week as we have a market on, we collect everything fresh on the day (cue 8am visit to the butcher and manic running around) and anything that's left gets frozen for our own use once home. I tend to sell a pigs' worth of meat plus extra sausages/burgers and bacon at one of our markets, but this will vary depending on your area/other producers doing the same thing.
If we've not got a market on we tend to get the butcher to blast freeze everything and just sell from the freezer.
HTH