First plug the arieal lead from your roof arieal into the socket at the back of your TV. this will elliminate any possible interferance from other divices.
A: if the picture is still snowy then the fault is reception, ask next door what their reception is like. If their's is ok you could have a faulty arieal or require a more powerful one
B: If the snowy picture is now ok, this indicates that the reception is good and the fault is interferance from another device eg video or sat receiver, games machine, ect.
Always use a scart lead to connect other divices to your TV if possible. if this is not then connect each divice individually to determine which one is causing the interferance. then consult the instruction manual for that divice to learn how to adjust the ( RF output ). After adjustment the TV will have to be retuned so it can recieve the new frequency from the device you have just adjusted.