It's the tire that maintains the shape- the tube just seals in the air.
Put only a little bit of air in the tube, not so much that it starts to stretch out. Place it in the tire, mount the tire on the rim. Be careful not to pinch the tube between the rim and tire. Once both beads of the tire on the rim, inflate it to about 15psi. Check all the way around the wheel to check that the tire is mounted evenly on the rim- there's a line molded into the tire that should sit on top of the rim. If it's not even, deflate, pull out any low spots, check to see that the tube isn't caught under high spots, and start pumping it up again. Check again when you get to 30psi- then 60 psi.
'''96 Litespeed Catalyst, '05 Litespeed Firenze, '06 Litespeed Tuscany, '12 Surly Pacer, All are 3x8,9 or 10. It is hilly around here!
I bought a pair of Nashbar's "Comfort" tires for my rain bike a while ago. They were listed as 700x32 so I also bought some 700x 28-32 tubes at the same time. Well, the tires actually measure 26 mm and the tubes turn out to be a stuff-fit and a pain to install. They work but are larger then needed and require a lot of care to prevent the excess rubber from being caught between the bead and the inside of the rim. It's fussy enough installing them in my workshop but it was a lot more fun repairing a roadside flat with one of them in a cold rain a few months ago. When these are used up, their replacements will be smaller.