as couple others mentioned...would DEFINITELY go with the 'short tips' (31 gauge / 8mm ) ... and i dont know if this is considered 'improper' technique or not, but i sometimes find it easier to go at the vein from an angle, like from the side, and rather than piercing the skin with the needle at a 90 degree angle, i usually go somewhere around 45 degrees or so, that way...when i get the needle in as far as it can...chances are good that i am in the vein, rather than pushing through the far side, which can occur if going at a 90 degree angle. having it in as far as possible will also help in steadying it and making sure you are in the vein, and STAY in while doing your thing...if needle is only halfway in or whatever, chances are good that while you are pushing the liquid out, the needle will move up or down, increasing the chances of leaving the vein.
and again, as few other posts mentioned...you will definitely notice if you are missing your shot or not. it will burn or, more often in my case...you will start to see a lump rapidly forming where the injection site is. if hitting on target, you should feel no discomfort, pain, resistance or anything like that..so if you DO feel one of those things, or see the lump forming...obviously you want to immediately stop and move to another site with what you have left in the syringe (thats also why i go slowly..so if i am missing..its less of the hit that 'misses' and goes to waste.
and one last comment...if youre new to shooting...you shouldnt even need to tie off as your veins are probably easily visible all up and down your arms and hands. there's definitely no going back either once youre shooting. i switched to dope about a year ago from pills..i always snorted the pills and for the first few months, the dope. but once the rush wasnt good enough anymore through the nose...turned to the needle...and yeah..DEF no going back from that, sad to say.
oh and lastly: switch up the spots you shoot...dont always use the same spot on your arm or hand.
good luck!