There are a few things you should consider when determining what RAM to buy.
1. How much you want. 16GB is a good starting amount for your purposes.
2. Type. You want the type of RAM that fits your motherboard. For a fairly modern system this is likely to be DDR3, DDR3L or DDR4. If you are using an Intel Skylake processor (i3/5/7 6xxx) do not use DDR3, regardless of whether your motherboard says it supports it. Get DDR3L or DDR4 instead.
3. Speed. This is getting a bit more complicated. To keep it simple, if you're getting DDR3 or DDR3L, get 1600-2133mhz, but don't pay a premium for anything higher than 1600. If it's slightly more expensive then go for it. DDR4, get at least 2400mhz. These are the sweet spots in terms of pricing to performance. You can pay a lot more for tiny improvements, but if you plan on doing this plan on doing more research into how RAM speed works first.
4. However many channels your motherboard supports, get that many sticks of RAM. Dual channel - get two. Quad - get 4. So if you want 16GB and your motherboard supports dual channel, get 2 8GB sticks, preferably matching. Make sure you put the sticks into the right slots if you are going to use multiple channels. Sometimes these will be colour coded, sometimes you need to consult your motherboard documentation.
5. Stick to good brands like Kingston, G.Skill, and Corsair.