EDIT: There's a great harm reduction text on the first page that I didn't look at before. Every must-shoot junkie-to-be should read it.
So my input doesn't really add much, sorry.
Well, everyone knows which vein I'm writing about when I write that it's the very first vein people aim at when they want to shoot (the one nurses take blood from the most often). It's vena mediana cubiti in Latin, so it would be something like median cubital vein, I guess. It seems to be an ideal vein for injection. CAUTION! There's a big artery over there, brachial artery near and median nerve. I've seen first-timers hitting brachial artery and mistaking it for median cubital vein because it's easy to see pressure in brachial artery (well, this one is also used for measuring blood pressure, no wonder) and if they use, say, a belt to purse then they hit the artery and even though the light blood gets into a syringe itself because of the pressure, they often push the plunger anyway. And this can cause even life-threatening condition. Arteries are deeper than veins but this one is big and very close to median cubital vein so I've seen many mistaken shots even though arteries are innervated and it hurts when one hits the artery. One doesn't have to inject to feel the pain, injecting is painful like hell.
How veins and arteries are located is a personal thing on the other hand but everyone has the same veins and arteries. The artery I'm writing about may be closer to median cubital vein or further. Well, one gets to know his or her body with time, I know my brachial artery is very close to median cubital vein on my left hand so I always watched out. Well, to be honest interest in anatomy helps here a lot.
If we're here to help, if it's harm reduction forum, then the best answer is (unfortunately only one because one thing I noticed that your best argument against shooting never stops a person who wants to do it and even if he/she considers leaving it at least for a second, he/she is going to do it anyway; I shot a lot of drugs, I had this opportunity to synthesize probably a lot over 200 active compounds, most of them being opioids, this is for research, I believe some mixed delta agonista/kappa antagonist might come out as an efficient antidepressant but as a junkie I didn't use the lab only for work and I shot ~100 of them, just opioids, you grab sophisticated books on synthetic stuff and voila, you know what you want but that's not the point... I'm not saying I did all of them all the time, most were synthesized once just to know how they work and I had my favorites, on these I was losing my life and even people who I had watch the lab and different drugs didn't see the point and I ran into some chemical mania because we did even small changes in structure, many drugs didn't differ in action, this is crazy what it does to mind, I even thought I could find better substitute than existing):
If you decide to shoot any substance (knowing from many stories and reports that this is the straightest way to get addicted to the substance you're wanting to shoot), you must remember that it must be done under very little angle (10 degrees would be perfect but 30 is enough) using a thin needle (25-30 gauge, 0,5-0,3mm, the best would be something around 27 gauge and 0,4mm). Once you hit the blood vessel, make sure it's a vein. Pull back the plunger slowly, if blood is dark red and it comes 'idly', this is a vein, when blood itself gets inside the syringe and it's light red, pull back the needle and apply fast pressure to where you hit and hold strongly for not less than 15 minutes, this was an artery.
If you're not experienced, if you've never injected anything intravenously, it's the best to have someone around to do it for you the first time and help you out (don't fool yourself you'll do only once and don't be mad at anyone if he/she doesn't want to inject you a drug, I wouldn't myself do it, instead I would take the drugs you have away and tell you my story - there are thousands of such stories).
To be honest, I don't believe there's some power to stop anyone who's willing to shoot a drug. Nothing. I managed to get just a few people out of this shit with methadone but they just had to see themselves how 'beautiful' it really is.