The first step in selecting a ram is to look at your ewes. Do they have any genetic flaws that you need to avoid in your ram? If so you need to keep this in the back of your mind when looking for your ram. Consider what your desired lambs will look like and focus on finding a ram that will fill that gap.
Are you breeding for meat or to produce more breeding animals? If your goal is to produce mostly butcher lambs you need a ram that is bred for that purpose, but he does not need to be purebred, or registered. If your goal is to produce more breeding animals, or those for show, and resale, you will want to spend more money and find a registered ram, ideally one who has proven himself at shows, or who has produced lambs who have done well.
The age of your the ram should be no younger than 5 months, by that time he will show you his potential, although he should not be put with too many ewes at this young age. You probably will not want to buy a ram that is more than 5 years of age if you intend to keep him for many years as fertility will decline after this age.
Look at the overall condition of the ram. Check his feet for signs of foot rot if it is known in your area. Examine his teeth especially if you are intersted in keeping his lambs as future breeding stock. It should also be said that a ram with small testicles should not be considered for breeding. Ask to see his mother. If she has lambed look at her udder; you should not buy a ram from a ewe that has a poor udder if you want to keep ewe lambs from him for future breeding purposes.
Be sure to ask about general health of the flock and if he has been vaccinated or wormed. Look at other sheep at the farm you are buying from, if you see any that are poorly looking, you may want to consider the overall health of the flock/farm as it relates to the prospect ram. If the ram is thin he will need lots of extra feed to stay in top shape for breeding your ewes.
If your ewes are smaller do not over match the ram for them or they will have a difficult time giving birth and you may have several dead ewes and plenty of bottle baby lambs to look after. Select a ram whose breed is comparable to your ewes, if they are first time lambers you may even want to select a ram that is a slightly smaller breed than the ewes.