If you look around BL (in the Other Drugs forum specifically) you'll find a billion threads on this subject, but I'll try to help a bit anyway -
Well, if you're right handed I'd recommend you inject in your left arm obviously - look for the veins near the crook of your elbow. If they're not visible enough, tie something around your upper arm (shit...forgot what it's called, someone help me out please), I use a belt and it works fine tbh. You can also clench your fist and that'll make the veins pop out more.
The important thing is to make sure it's indeed a vein into which you're injecting, and not an artery (look up a diagram on the internet). You'll know if you've registered into an artery because when you draw back on the plunger, the blood will be very bright and it'll come out with a lot of pressure. It'll also burn a lot. If you see this, do NOT proceed with injecting as it could result in a loss of limb.
You'll know you've registered when you pull back on the plunger and a little blood comes into the syringe. And then well, just push back. Don't really know what else to say, good luck, be safe, and remember to never re-use the same needle.
Oh wait, just found the IV megathread. You should check it out -
http://www.bluelight.ru/vb/threads/...ation-MEGATHREAD-and-FAQ-II-show-me-the-blood