I too have been through the scare of a lump, although in my case it turned out to be a massive abscess (indeed, my GP was so worried by the size of it that he phoned me at home to double check the information I had given him and said he was VERY concerned, which meant as far as I was concerned I was a gonner already).
You will most likely have an ultrasound or mammogram, a physical examination and possible needle aspiration or a core biopsy on the lump.
I was constantly in fear of the worst case scenario, to the point where I was pretty much planning out what I wanted to do with the time I had left, the whole week between attending the GP and going to the clinic was the most surreal, terrifying and yet life affirming time of my life thus far.
When I was actually told what it was, it took me ages to take it in and actually realise that it wasnt cancer. I ended up having an op to clear it, and I was on a ward with three other women, all of whom DID have cancer and who were some of the most inspiring people I have ever met. They were so matter of fact about it, so positive, and so well adjusted that it just floored me, to the point where I felt almost ashamed to have been worried about what was going to happen to me. If they could deal with what they were dealing with so well, then I had to pull my socks up and stop being a total idiot about my fears for my own op.
As others have said, 90% of breast lumps are benign and this put the odds firmly in your favour. But if for any reason it is not, then know that you will find in yourself an inner strength and power that you never knew you had to fight and beat it, and that you have the best medical support you can possibly get. Those guys are AMAZING.
Good luck for your tests!!