What to do if i had a dream about shooting up, what does this mean?

Nothing: Hopefully you have enough sense not to do needle drugs in real life. This is a random image that your brain generated. The fact that it bothers you is a good thing -- injectable drugs are a fast life-ruiner.

Dreams about problem: solving/experimenting within our own more deeply internalized virtual reality about what we have been doing, what we might do, how it might feel, how it might work out when awake. Life is complex and our primary motivations stem from an integration of life experiences & feeling understandings of reality, as we view it. The dream clearly represents ideation about the issues&pros/cons in your view.