I actually just had my breast clinic appointment today.
I found my first lump in my breast when I was 19. Like you I was a bit frightened and was referred to a consultant at the hospital. Basically they feel your breasts, locate the lump and just see how it feels to touch. They can get a very good idea of what it is from touch alone. I then had an ultrasound, and they were 99% sure it was harmless but I also had a biopsy. Which does hurt a little but is over very quickly. I think the results took a couple of weeks, but there were very reasurring at the hospital so I wasn't too worried at this point, although it did play on my mind every day until the results. It came back as being a fibroadenoma
Fibroadenoma - Wikipedia, the free encyclopedia). Which are harmless, benign lumps. They move quite freely under your skin btw, so if yours is like that it's a good sign.
To cut a long story short I am now 26 and have several of these lumps in both breasts... apparently I'm quite a rare case and they keep appearing. I now go for a routine check-up with my consultant every 6 to 9 months, just to monitor them as I am a bit of a one off!!! In the UK they only remove these lumps if they are 30/35mm (forget which), my biggest is 24mm.
The real downside for me is I find it harder to monitor changes in my breasts and the lumps as I know have so many. I may look at getting the larger 3 removed privately.
Besides fibroadenomas there are other things it could be that are totally harmless eg cysts, swellings caused by infections etc etc. Chances of the lump being cancerous really are very slim and that's what you have to remember.
I hope your appointment goes well x x