If you are unsure of which shoes to get then you need to follow a few quick guidelines.
Firstly, look at you old shoes and see if there are any signs of them being too narrow (splitting at the sides), too short (big toe poking through, too wide (the laces closed up) or too large (wear on the inside of the shoe ).
Secondly, see if they are breaking inwards or outwards, indicating you need pronation shoes or neutral shoes.
And finally, look at the outsole wear on the shoe. Most runners wear the outer edge of the heel then the middle or medial part of the forefoot. This puts them in most categories but if the wear was on the outside of the shoe all the way down then a neutral shoe would be best. And if the shoe is seriously broken inwards then a motion control shoe is needed.
Also take one look at your foot on the ground and see if your ankle moves inwards when bending your knees (pronation) see if your arch is flat (a straight lasted shoe) or if it is very high and stays up (neutral)
This is checking at its very simplest and the rule of thumb is for the shoe to not squash the bones of the forefoot together and for the big toe to be one cm off the end of the shoe when sitting down. This is generally reckoned to be half a size bigger than your normal day shoe.