Sources of Fuel
When you are out in the woodland, you will find plenty of wood to use. Best not to use trees that are still growing, as that would be detrimental to the eco-system. Use the dead or fallen trees, limbs and branches. Some may be rotting on the ground, which will be of no use. But there will be plenty of useful wood scattered around.
Here's a short list on the good and not so good. Some of the trees will be in the section on trees to help you identify..
Good to use
Oak is a good, it burns long and hot.
Ash. A good burning wood with steady flame and good heat.
Birch all types, it burns quick, so have a good supply to hand.
Hawthorn is a very good firewood, it burns hot and for a long time.
Maple is also a good all rounder.
Pine is good also. Cracks and pops and does smoke.
Hazel, a fast burning fuel, you will need a lot of it.
Not all these trees you may find in one area, but it is good to get to know what you are using.
Over time this will made your experience much better as you will know what to look for.
Not so good to use
Willow must be dry, burns slowly.
Chestnut has little heat.
Douglas fir, I've not tried this, but research shows to avoid it.
Alder and Spruce burns too quick.
Poplar makes lots of black smoke, best to avoid.
Again the more you will learn on the trees, the better you will be prepared.