Bike tubes are pretty flexible and have a lot of room for error, but why not get it right to begin with? Similar to tire size, you'll pick the diameter and width that's appropriate for the tire you're using. Don't sweat this too much, inner tubes stretch so much it's quite possible to inflate a 29" tire with a 26" tube in a pinch.
The valve can be a little tricky. Look at your rim and see if it's drilled for a schrader or presta valve. You can stuff a presta valve into a schrader drilled hole, but not the other way around. It'll be a bit loose fitting, but your wheel should come with an adapter that takes up the slack. If you're using a presta valve, the length of the valve you're using is somewhat important too. Most valve stems will be in the 40-50mm range and will clear the vast majority of rims. If you're using super-techy-aero-deep-v rims, look up the rim depth on the manufacturer's site and make sure the valve stem is an extra 5mm or so to clear it.
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