The most safe area to shoot in the tri is the long head which can most easily be located by doing a bicep pose and marking or just noting the lowest point where your tri hangs. This should be more or less the complete opposite side of your bicep peak. Then you want to straighten your arm slightly, but do not extend it fully or your tri will be fully contracted.Basically you want your forearm to be at about a 45 degree angle, so your bi is still semi contacted and your tricep is more or less relazed. Then inject in the marked site. The reason you want to avoid injecting into a contracted, straightened arm is that the inject will be more painful and can result in a backflow and leakage of the oil.
I recommend using the smallest gauge possible. Usually you can get away with a 25 gauge depending in the viscosity of the oil you're injecting. A 5\8 inch to 1 inch needle is the optimal size for the tri. hope this helped bro.