How can i tell whats wrong?

Driving to work today and the car was lacking power

, when changing up a gear it felt like the engine was breaking (not sure what technical term to use, basically when u change down gear to early and the engine slows the car) but then the power cam back a bit, but when taking my foot of the accelerator it did it again.
When i got to the car park at work and slowed down the car would not pull forward as normal, then when stopping the car would pull of at all. I had to rev the engine high to move.
The car has problems starting when wet, but started fine this morning, currently the petrol is low but it has been lower before with no problem, and when checking the car started fine at work so dont think its a fuel problem. I have checked and the petrol filter has petrol in, i have checked the oil and that is fine.
Does it sound like the car is mis-firing? and how can i tell? When i get a chance what should i check? plugs, dizzy cap. or does it sound simple like low fuel?
Cheers in advance.