OK, lets just set something straight here:
For a constant volume if you want to increase pressure you have to:
A ) add more gas (ie additional molecules); or
B ) increase the temperature.
For a given constant amount of gas (note that i said AMOUNT, not volume):
A ) to increase pressure, you reduce volume or increase temperature
B ) to maintain a constant pressure in a reducing volume you have to add more gas or increase temperature.
As MattSR said - PV=nRT, where P is pressure, V is Volume, T is Temperature, n is the amount of the gas in moles and R is a constant.
Does this clear it up?