it's probably just a little speck of dust.
first, try a blower. I use this one:
if that doesn't work, buy a cleaning kit, I use this one:
There are other cleaning kits that come with solution and swabs to clean your sensor. I personally don't fuck with that as I'm not trying to leave behind any residue - I take mine into a camera store that I trust if it's that bad that it needs that.
I have two specks on my sensor right now, I'm just hitting them with the healing brush for the time being. It works fine in most cases. Sensor dust happens.