Ideally, you should take your violin with you to a music store to test several products. You also may want to ask your violin teacher or violinist friends for their recommendations. Some violin players use no shoulder rests, others prefer a particular brand, and some teachers recommend the use of round, make-up sponges (see pictures below for samples).
To minimize neck abrasions, some violinists also find it helpful to either place a scarf or cloth over their chin rest, or to use a chin rest cover.