It probably isn't that hard, and there are lots of articles on the net about tank building. But the main reason I wouldn't is cost. For the raw materials it would cost me about about £100 to build the tank i've got, and you could buy one for about £40! If you can get a good deal from a local fish shop you're better going there. You're also more likely to get a good deal on the equipment and stand if you want one. I got a second hand tank for only £20. But then add on the cost of the stand, the filter, heater and lights separately, i could have got a much larger tank in a deal for much less than i spent on this setup.
That said, I made the stand myself, noone else is going to have a setup like the one i have, Unlike the thousands of jewel or aqua one aquariums.
A bit of advice i would give is to start slowly. Give your filter time to mature for a good week before adding a few hardy fish. Have a look at fishless cycling also. I didn't and really put my guppies through some nasty water conditions while the filter was maturing properly. I now have the stock you see in my signature, and have only lost one neon tetra so far.
You should also find as many fish shops as you can in your local area, vist them a couple of times and decide on the ones that are going to give you healthy fish and good service. Talk to the staff and see who is going to give you good advice on your community tank and who is going to happily sell you a small oscar that will outgrow your tank in a year. And finally, get a test kit, even just dip strips that you stick in the water. This will let you know when your tank has cycled, and also will warn you of any problems you may be about to have!
Good luck!