Celt gave you two of the best site for injecting.

the top/outer thigh is the best location to do it for sure. to find the best spot take your index and your thumb, spreak them wide apart. put your index on your knee take mental not of where your thumb is on your quad. do the same from your hip. that should give you roughly 3 inches, that's your target.

If you hit a vein, and at some point you will at least once it's nothing to worry about. for those using a 1.5" needle you can just pull the needle out a bit and inject away. I use a 1" in the quad and if I hit a vein I rather just pull out, change the tip and start over, same thing if you hit a nerve (those will make you jump the first time for sure)

best of luck bro
Arush