Um...okay, that's pretty vague, but here goes...
1. First and foremost: honestly evaluate whether or not you are in a good financial situation to buy and provide care for a horse. Its an expensive hobby and you are taking on a huge responsibility as the caretaker of a living creature.
2. Measure your skills in working with horses. Overbuying will make for an unhappy you and an unhappy horse.
3. Think about what your long-term goals are and buy a horse that will help you achieve those goals. This may influence price, age, breed, size, and certainly conformation.
3. Its very helpful to find a purchase partner: someone with good knowledge about judging horses and an emotional detachment from the purchase, who can give untarnished and honest advice.
4. Start close to home, if possible. Transport can be an expensive prospect in many cases.
5. Don't buy a horse unseen or untried. Not everyone adheres to this, and in some situations its impossible, like if you are buying a tb fresh from the track, for example, or have decided to purchase a foal in-vitro.
6. Always have the horse vetted before you buy.
7. Receive a guarantee from the buyer that they will provide the registration papers if the horse is registered.
8. It never hurts to haggle. Some sellers will be very firm on the price, but you never know until you try.