Greetings, Stranger. Glad you found the “message in a bottle”, pitched to the tide by the Woody Dwarves. Beckoning you here, to the Tiny Forest.
Ok... let’s get some preliminary “mumbo jumbo” RIGHT out of the way.
You are going to want to update your profile to reflect a location or climate zone. (It can be vague, mine just says, “Northeastern Wisconsin”) this way advice given will be pertinent to YOUR situation... ‘tis a large world.
What trees do you have already?
Where are you keeping them?
Seed-growing is a very long, TRULY rewarding process... it will be important for you to get yourself some OTHER trees to work on... or you possibly may “love” your seedlings to death.
What is the area you will be “doing plants“ in?
A “potting-soil”-type substrate will work fine for sprouting seeds.. but it is not BONSAI soil (solely bonsai soil isn’t the greatest for germinating seeds, from MY experiences)... Bonsai soil is.. different.. most people highly-inorganic, mineral particulates as soil... Most people have what they PREFER (usually what works best in their climate, with the amount of care they can provide)
Many mix their own (Myself included) and have their own recipes and ratios geared towards certain needs.
Run a youtube search for “Bonsai Soil”.. there should probably be some good information to “in/digest”... but normally.. EVERYONE would answer your “Soil” question at LEAST slightly different.