Make sure you first do your research on the sort of boat you want. This is often the hardest part when buying your first boat as it's difficult to know exactly what you want until you start sailing. Boat shows are great for new boats but of little help in gaining knowledge on models that are no longer current.
Make sure you look at owners' association web sites for the boat you are interested in and on these you can normally post a question.
Another great way to get knowledge is to simply walk around a boat yard or marina and speak to owners who are nearly always more than happy to talk about their boats.
If you can, charter the boat you're interesting in for a weekend (or longer if possible) to get some first hand experience.
Surveys
It is advisable to have a survey done. Osmosis or grounding damage may be impossible to spot yourself and you may end up buying a boat that will cost you thousands to put right.
Many insurance companies want to see a survey which is another reason to have one done before you buy unless the seller can give you one done with in the last year or so.
What do I need to do once I've bought a boat?
Insurance
Insurance for your boat is not compulsory but law but it is extremely advisable and is and can be a requirement for boats using certain marinas or harbours. Boat insurance is generally far cheaper than motor insurance for a far greater level of cover. If you had a survey done on your new boat make sure you hold onto it as the insurance company will almost certainly want to see a copy.