Hello and welcome.
Try googling on "try caving" and it should take you to a site of that name which has lots of info on starting out, and the names of clubs you could join which would welcome new members. And have a look at the other threads in this section, if you haven't already.
From Doncaster you should be in reach of clubs in the Sheffield area (going on my rudimentary geographical sense which says "Doncaster = S of Leeds therefore =Sheffield)- most clubs have members from quite a wide geographical area. White Peaks and Yorkshire Dales are both good caving areas.