You need to look at the following things:
1) Latency - Timings (lower is typically better)
2) Speed - MHz
3) Voltage - What is your voltage and does it meet your CPU's specs (Intel i3, i5, i7 less than 1.65V)
4) Manufacturer - Is it a good manufacturer like Corsair, Mushkin, G.SKILL..
5) Compatiablity - Is the RAM on the QVL list for your motherboard (not a must but is always nice!!)