Where have you been trying to shoot? A lot of people who don't really know how to shoot their shit start with trying on their lower arms and shit and that's not always the best place, those veins can roll easy, plus they bruise a shit load.
The best bet, which has ALWAYS worked me (I have awful tiny/shitty veins) is right in the inner crook of your arm/elbow. You won't see the veins usually but flex your arms a few times and feel around. Half the time, feeling out a vein is more effective than seeing one. When I would bang my shit at first and had a hard time hitting I would do like 10 pushups before hand and my veins would be poppin and much easier to hit haha. I wouldn't even ever have to tie off either. Actually I've never tied off really. Taking hot showers can help too. Also, you must angle the rig right when trying to hit. Going straight up at a 90 degree angle won't work usually. Try and go almost paralell to your arm or at like a 20-30 degree angle.
Your best bet actually, if possible, if always have some one who is experienced with it help you for your first few times. I know I was always nervous at first so my hands would be unsteady and shit. After a while that shit comes so second nature. I would hit within .1 seconds of the rig entering my arm a lot of times and I wouldn't even see any veins or feel around, just knew the spot exactly.
Good luck though, and I hope you know this is going to make things A LOT worse and more difficult. Like me, and a lot of people, after a while you'll be more addicted to the needle and the process of cooking it up and shit, then the feel of the dope itself. And the high that you like now from snorting it? Fuck that. The actual duration of the high will be forgotten cuz it starts to be completely about the rush from now on.
Btw, rotating injection sites makes it a lot easier. And just remember if you hit, the blood will just explode into the rig without even pulling back the plunger. And if you miss, you'll know cuz it stings like a bitch. If you push down and feel any bit of stinging, stop.
good luck dude.