Thank you for your query.
You don't mention whether this lump is tender or red but I assume as you have been pressing on it firmly, then it probably is not.
The most likely diagnosis is a sebaceous cyst
. This may be related to acne
, which at your age, is not uncommon.
It could possibly disappear over time. It may become infected. My advice would be to wash the area as usual. Do not keep touching it as this may cause infection. If it doesn't reduce in size and/or it becomes infected consult your doctor for further advice. It may be appropriate to try antibiotics if there is infection present.
I hope this information has been helpful.