To begin with it all depends to a great extent on what you require a tractor for, what you are going to use it to do. It may be you have a very substantial area of land that needs to be mowed on a regular basis, or perhaps you need a machine for plowing, or harvesting? How large an area are you dealing with? How often will it be used? What attachments will you need, as this will dictate which model you buy.
You will also have to decide on a gas or diesel powered tractor, and again the specific uses and size of the land area will perhaps help decide that. Many of the tractors to be used on smaller areas are gas, but the models designed for large industrial sized areas will usually be diesel.
As mentioned, there are many types of tractor, starting with the lawn tractor which will be used for mowing a lawn and not designed for anything else. Such machines can be relatively cheap, and will come with a gas powered engine, are designed to be used almost exclusively with a lawn catcher attachment, but some of them will even have the capability to be used for some heavier duty tasks. Such machines are useful but not up to the sort of tasks more expensive tractors can be used for, and will not last long if put under that kind of strain.
The next larger form of tractor tends to be one that can be used for various uses but still on a smaller scale, such as plowing, mowing, cutting etc. There are tractors which will do such tasks which are classed as garden tractors, and this is often just a lawn tractor but more substantial and with the ability to pull heavier attachments. More useful might be a compact tractor though, which is more like a full tractor with all the capabilities of the larger cousins, but smaller and very much alike to garden tractors. They can do almost everything, mowing, cutting, loading, digging, tilling, plowing, hauling, and are substantially built, with full diesel engines and will last for many many years. Naturally though such a machine will cost considerably more than the smaller lawn or garden forms.
At the top end of the market are the professional farm tractors which are used on large farms and can be used for substantially any task, small to large. Such a machine will cost a large sum of money but will be a tractor to keep for decades, potentially a farm would only ever use one with purchase of occassional parts and attachments. Keep in mind though that the attachments for a large from tractor will themselves cost more than small tractors.