You may well find that John Thomas Forster had no known father.
With his exact birthday, 19.2.1891, there are only two possible children registered, one in Medway, Kent and the other in Gateshead. He is probably the Gateshead one, in which case there is no mother's maiden name given, i.e. his mother was not married.
Looking at the 1891 census for Gateshead (taken on 5th April) there is a John Thos Forster, 6 weeks old, living as grandson to John Forster, 62, widower, coalminer. This John has two unmarried daughters with him, Margaret 27 and Elizabeth 21 so either could be his mother.
It is worth bearing this in mind when you get the marriage certificate as people often put the name of a step father or just made up a name if they didn't know a father's name.
Good luck with your search