First of all, I am sorry to hear about your Grandad's passing. Many of us on the Forum learned so much about growing from our grandparents, and you may find you have taken in more than you think. And what a lovely greenhouse!
The first place to start is to please let us know if there are any plants in there, and if you know what they are, so much the better. Then we shall be able to make some suggestions which will hopefully be helpful