HELP-ADVICE NEEDED-SHED VARNISH AND MOULD

[Leicestershire Lady]

I had a quality shed last year. I varnished a shed once and loved the finish so decided to do the same to the new one. I got my varnish from a large DIY store after asking advice in the store about which product to use. The shed was given one good coating and looked good. The empty can was thrown away so I have no idea what product it was. Over winter the shed has got stained with mould which appears ingrained. It looks awful. The walls inside look ok. The contents of the shed are mainly kept in large plastic tubs. These tubs have mould on the outside. Contents inside tubs are ok. I have a tent in there which is in a canvas bag, that has mould on it too. Some metal has mould on it too. The shed is not got a felt roof. Its corrugated, not sure what its made of. Fibreglass or reinforced plastic, bitumen??? Just not sure. There is also air gaps where the walls meet the roof.
I have no idea what to do now. It needs cleaning. How ? what products to use? Should I then re varnish using 2 coats? Will different products go over varnish? I am wondering if this job is to big for me to do.
can someone please help and give me some advice