Hello and thanks for your post,

Finding a lump can cause a lot of worry but the majority of lumps particularly at your age are harmless (not cancer). It is rare to get breast cancer when you are 18. We can't diagnose you but it is far more likely to be a fibroadenoma or a cyst which you can read about here

Breast Cancer Now also have some information on lumps when your younger which you can see here

You may find that when you have your period your lump may disappear so you may want to wait to see if that happens. If it is still there after that it might be a good idea to make an appointment with your GP so they can do a breast examination and hopefully put your mind at rest. You can always ask to see a female doctor if one was available. 

I hope this helps.

All the Best,

Catherine