From a new guy who has experience with IM injections…
Injecting air is not a bad thing when doing IM injections. Having an air bubble in the syringe is like the “Air lock method” of injection, where all the aspirated air is injected back into the site, essentially creating a barrier of air where the compound won’t leak right out of the hole you just created with the needle. So in that instance, already having a small air bubble in the syringe would not cause any issues.
One of the the most important things to do, IMO, is to aspirate the site prior to pressing the plunger in. You basically stick yourself, then keep the syringe steady with one hand, while slowly puling the plunger up with the other. If you see blood, that’s not good - do not proceed with the injection. Slide the needle out carefully, switch to a new needle (just twist it off the syringe, since it’s already loaded with your compound and twist on a fresh one), and pick a new spot. I only have experience with quad IM injections, so no advice for glute or site injections.
Sanitation is really the other major player in this game. Swab your site and swab the bottle top. If you need to switch sites, swab again to be sure. And for the love of all that is good and holy, let the alcohol dry before injecting. The needle will burn like hell if the site is not totally dry. It should go without saying that the syringe/needle should stay in their sterile packages until the last possible moment, once you remove the cap from the needle, make sure the needle itself only touches the bottle top or injection site. My recommendation for minimizing pain in IM injections is to draw the compound up with one needle, then switch needles for the injection. It’s more sterile AND the needle going into your quad will be sharper which will equal less pain/discomfort. Dull needles hurt, badly.
Another poster already talked about potentially “bad” gear and not knowing what is in the compound you are injecting. If you are planning on injecting test, my advice would be to see if you can find a doctor willing to prescribe you test E or test C or test sustenon for IM injection. Granted, it may be extremely difficult to find someone willing to do this for you, and you should be very well versed in the types of labs you will want to have performed on a regular basis to monitor your overall health while reaching your mass goals. But this is not an impossible feat.
Best of luck.