Check out this BBC website for the fundmentals about forces.
http://www.bbc.co.uk/bitesize/ks2/science/physical_processes/forces/read/1/
http://www.bbc.co.uk/bitesize/ks2/science/physical_processes/forces/read/1/
The following BBC website goes into forces in more detail - Check it out!
http://www.bbc.co.uk/bitesize/ks3/science/energy_electricity_forces/forces/revision/1/
http://www.bbc.co.uk/bitesize/ks3/science/energy_electricity_forces/forces/revision/1/