when i ran my circuit last it was rapidly gaining tempurature. It got very hot in less than a minute and it was using anbout 0.9A. Why could this be then?
You should use a PNP/NPN transistor pair to drive a MOSFET. When you use a mosfet it will have much less on resistance than a NPN transistor, and it will then have much less heat dissipation.
0.9A from 5V is only 4.5W. The fan probably used most of it.
And as I said you will need over an amp of input current to get like 200mA of 12v on the output, and as far as I know, most 12volt fans use about 2.4 watts, so you would expect maybe a power draw of around 5 watts at 5volts.
-Ben