Tubes are quite forgiving, and most will cover a range of sizes. You don't want to go too small, and you you don't want to go too big. Any tube in the 1.8-2.2 range in 26", and with the right type of valve will do.
an intertube.
40 psi
Yes you can.
Of course you can.
MAYBE - , frame size has nothing to do with tire size. Style does, and if its a mountain bike, it will not work with a 27 (road racing) wheel and tire. Brakes will not fit.
In 1979 the first official mountain bike was launched. Tom Ritchey of Redwood city, California started building fat-tire frames. The Lawwill Pro Cruiser was the first mountain bike which was sold for $500 overseas.
Several websites offer good wheel and tire packages for mountain bikes. Try Performance Bike, Rocky Mountain or Wiggle. The Halfords website is also excellent.
motor bike or bike bike? I have tires on my Mountain bike which run 50-60 each. but they are for downhill and are pretty top end.
The hardness of the rubber. Softer rubber gives better grip on rock, but wear faster.
In order to find the proper replacement for your bike tire you need to measure the wheel and determine which type of tire your bike uses. You can also take it to a bike repair shop and they can help you pick out a replacement tire.
It depends what use the bicycle is put to, it may be a racer, a road going tourer or an off-road mountain or trail bike. All have very different tires.
You can find a bike tire by visiting a local bike shop or even a retail store like Walmart.