If someone is on your frequency they will interefere with each other. If you are flying your plane and someone turns their transmitter on you will most probably lose control. Most clubs operate a scheme to prevent this. It is often in the form of a 'peg board' where there is one peg for each frequency. If you go to pick up your peg and it is not there it means somone is on your frequency and you will have to wait till they're finished.

Frequencies can be changed by changing crystals. These are not too expensive.

Which radio are you planning to buy? Have you decide what model you want yet?