The First things to do in the Greenhouse at this time of year, is to Disinfect the interior!
There are Insects and aphids that can stay in a Greenhouse over Winter, which end up infecting plants & seedlings as they germinate & grow. So use a Spray that is recommended for this work.
Don't forget to wear a mask!! Otherwise your eyes and mouth are liable to get splashed.

Afterwards you can start sowing seeds. But make sure the trays & pots are also clean, as they carry disease too. Any Multi-purpose compost is fine to use for seed sowing.
The Greenhouse will need Insulating against very cold weather, until around April, when the plants are growing well enough. I use Paraffin in a heater to keep the Greenhouse just warm enough. As well as the Polythene insulation too.
Late January you can begin sowing seeds like Lobelia & Geraniums. Though Geraniums need an early germination, with heat to get them started. So the trays need a cover or Cling film over the tray. Seeds only need to be just below the surface too, then watered well.
As long as the pots or trays are kept moist and warm, they will germinate in a few days time.
But don't over water! This can make the compost too wet and the seeds will rot away.
I often sprinkle a layer of perlite or silver sand over the trays after sowing, to stop green algae spreading on the surface. To cure this, water the trays/pots with a Fungicide, diluted with water. Many seeds can be sown from January to March/April.....suitable for Summer flowering. On warmer days its best to open windows in your Greenhouse too, for air circulation.