Best seed starting soil for mini greenhouse?

[3riverschick]

I am going to try growing a bigger garden this year. Encouraged by the success of my sweet peppers and chives from last year. (bought seedlings). I have some mini greenhouses with clear plastic domes I got at Dollar General (DG) last year at end of season. Also have one of Tractor Supply's small greenhouses. The one with wire shelves and the plastic walls/roof. Have one DG Mini Greenhouse sized seedling heating mat. I can't find a recipe or product name for the best seed starting soil mix. Can anyone help? I am hoping for something organic.

I really don't know what I am doing, just gathering everything together the book says I need to start seeds. Does this forum have a thread for absolute beginners in seed starting?
Thanks so much for your help!
Best,
Karen in western PA, USA , Growing zone 6B
at the foot of the western side of the Laurel Ridge of the Alleghenies. 15658

 

[lazy gardener]

I do a lot of seedlings, so buy a bale of Pro-Mix. I love it and a bale will last me 2 - 3 years depending on how many seedlings or potting I do. economically, I don't think you'll find a better deal if you have somewhere to store it. I leave it sitting outside my garage on 2 x 4's, under the overhang, so it doesn't get wet. I just need to remember to bring it inside before I need to start seedlings so it will thaw out!