Info do's and don'ts about injecting

[Trouble72]

I am an amateur, I only know one place to 'use', yes the arm, and I wasn't even taught how to do it there, but anyway, I've been doing 'ok'ish' there, but have been using often now, and have had a few misses and times I just couldn't find the vein,it's starting to swell and is getting tender, any advice anyone? pls