It's just up to you really. You can buy one already assembled. Or go to a place like Del-Ton.com and pick each part from a list (different stocks, barrel, etc) and they will build it for you. Or you can buy each piece and assemble it yourself.
From what I have seen, I think the cheapest deals are on the pre-assembled rifles. If you don't want to know how they are made, this may be the best route for you.
Ordering one assembled the way you want it will give you more options but will probably cost you a little more because you will start picking better parts than the pre-assembled rifles have. And you still won't know any more about how they are built.
I'm a person that likes to know how things tick, so I enjoyed putting one together. It's not real hard. How much you want to assemble will dictate how many tools you need to buy. When you start replacing barrels and checking head spacing your an AR junkie.
You may wish to consider this. Decide what you want the rifle to look like and what sights you will want on it for the shooting you will do. (long range, short range)
Buy an assembled upper. That way the barrel, handguards, gas block and tube, and upper reciever are already together. Then buy a good bolt, carrier, and charging handle, to put in the upper and check the head space to make sure it is correct. (a gauge is less than 20 bucks)
Then buy a striped lower, a lower receiver parts kit, and a butt stock. That way you can learn how to assemble a lower and know how to replace a trigger. You will probably want to polish the trigger or replace it with a RRA 2 stage later.
You may not save any money this way, but you will learn about bolts, and lowers. You won't need any special tools. And you can always start replacing barrels later if you want to learn more.
If I'm not mistaken you are allowed to build one rifle (lower) a year. You will want to check on that if you want to do more than one. Sorry I do not know more about that.
Hope this helps and I didn't forget anything. :wink:
TA