As above, follow tabethas advice, it's the best place to start, are you sure it's not just a knackered battery???? The solenoids do stick on the starters, i bought a new one and left in for around 2 years, it was stuck when i came to eventually fire the car up.
Have tried it using he battery of my daily drver to (2.9 Xr4x4) still no joy
Have also tried wiring a direct +ive to the wire actually entering the stater itself and still nothing.
My only other thought is that i cna try using my battery charger on it, set on 12v, i will be sure of good power then.
It might be that my engine is stiff from being sat for 18months and the force of trying to turn the engine has cooked it, is the best way to see if this is true to take the plugs out and use a socket on the crank bolt and see if it moves?