Transformers that convert mains voltage to lower voltages are much used items - especially in low voltage lighting. There is an incredible amount of makes and models to choose from, when shopping for a transformer, so I want to give you a few tips to take with you:
1 - Make sure the transformer outputs the low voltage you need. A very common type is the 12v transformer (one that outputs 12v) but many others voltages are available. You won't do any damage if the voltage is lower than required for your purpose, but but it might not work very well. On the other hand, if you feed 24v to something that needs 12v, you might damage the electronics severely, so this is something to get right.
2 - Know the power rating of what you want to power. If it is halogen light bulbs, figure out how many watts are consumed by each bulb and multiply that with the number of bulbs you will be using. That is the minimum power rating of the transformer you need. A good rule of thumb is to have a little headroom in the power, meaning that you might have at least 50% more power available from the transformer than you think you need. If your light bulbs consume 100 watts, then get a 150 watt transformer at the very least. Yo never know if someone in the future will put more power hungry bulbs in the system, without taking the capabilities of the transformer into consideration.
3 - Be sure on what kind of voltage you need - AC or DC? A transformer alone can only output AC voltage, so if you need DC, what you really want is actually what is called a power supply. This is a transformer plus some extra electronics. Also, do you need bipolar or unipolar outputs? A bipolar transformer has three output pins, which are called the positive, the negative and common (ground). A unipolar transformer only has the positive and the common - this type is what is normally sold as a low voltage lighting transformer, among other things.
4 - Also, if you want to be able the dim your low lights, you should know that you need to look specifically for a dimmable low voltage transformer, as not all types will allow this.
Finally I just want to say a bit about safety when working with electrical installations. The best thing is of course to get a trained electrician or technician to install your transformer, but a lot of people have done it themselves with success. - except for one thing: The actual connection of the transformer to mains voltage should definitely be done by a professional. Mains voltages are lethal, so stick to working with the parts that carry low voltages and you should be fine.
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