-just ride as much as you can and have fun
-try to ride your bike instead of a car or public transport
-on your weekend try to make a long bike trip, can be slow, can be paused, just ride somewhere (coffee place, nice place outside your city…) and every week try to make this trip longer (without carrying too much about speed) i know it’s winter but winter in UK is like summer in other countries and i bet you have lika a mega cool bike coffee place in every city
-90kg is much more about eating than training or at least 50:50 so an app to count calories like myfitnesspal is a good start, just to figure out how bad you eat or what you can change, ofc by riding you burn a lot of calories but you have to eat more to fuel your rides.
-install strava (fb for training people) and check cycling groups near your place, sometimes it’s much better to just go outside and meet similar cycling/running people (many of them, even the fittest ones, started from worse couch potato condintion then you and are forgiving for new people
-trainer is great if its cold or wet, you can start just by riding with low intensity (like you feel that you are pushing your pedals but still can breath and talk ) and make it 1h to 3h long, buying HR strap would be great
-make ftp test and set your zones and besides long slow rides (Zone 2 ) you can add some intervals twice a week
-i know that you wrote you want to ride indoors but if you sit a lot in front of a screen and want a life change, going out is much bigger change that sitting on a trainer inside, bike is a great oportunity to get outside for some endorfins, meet new people, feel some wind in your hair and just grab some fun from life
-by simply riding you can improve a lot and meanwhile gather some more info about basic intervals and training plans
ps. remember to have a strong fan when cycling indoors!