If the water heater that you presently have was handling the hot water requirements properly, record its capacity and run down to the local Home Deot/Lowes etc. and get a replacement that is the same size. Try to make sure that the vertical dimensions are the same, if it is a gas heater. If electric, no problem there. Grab a pipe cutter (if you do not have one), teflon tape and a couple of flex lines just in case, you can always return the items them later if not needed.
If the old one is not working, disconnect it before hand and start to drain it.
You should be able to get the new one installed by yourself within 1-2 hours.
/edit
Did not pick up part on gas heater.
Also, double check the building code on heaters. They may require an 18" space from the floor which may not exist when unit was originally installed. If required and you do not have it, I found that the easiest way to build up the platform is to use some concrete cinder blocks. Cheap and stable.
This will also then have an affect your flue. You may have to adjust a joint or two.
Good Luck