It doesn't take much to grow plants from seed, one thing it does require is potting mix (aka "potting soil"), here is everything you need to know about it.
Some people call it "potting soil" some people call it "potting mix," because it technically is not soil. Soil is a rich mix of clay, rock and organic material. Potting mix is an immitation of soil, but it is meant to be light and sterile.
Potting mix is used to grow plants in containers, and it is used to start seeds. I'll describe the most common ingredients used in potting soil, then give you my recipe that I use pretty much every time.
The Base Material
If we are comparing potting mix to real soil, I'd say the base material is a replacement for clay. Think of it as the meat of the soil, that the plant's roots are going to grow into. As the plant grows, this substance is going to give it something to grip onto.
Your two main options here are peat moss and coconut coir.
Peat Moss
Peat moss used to be the most common base of potting mix. It is still frequently used in commercial potting soils, but coconut coir has been taking over.
It is highly absorbant which is what has made it convenient for this use. Peat is also somewhat acidic. This is sometimes used to advantage, when growing plants that require acidic soils. But most plants do not need the acidity.
Peat moss is made of a decayed plant material formed in bogs. The U.S. gets most of it's peat from Canadian bogs. These bogs grow at a rate of one inch every fifteen to twenty five years.
Entire ponds are harvested by machine, often beginning by draining and drying out the ponds, then packaging the material. It is devistating to the ecosystems that rely on it and it will take thousands of years to regenerate.
Coconut Coir
Coconut coir has begun taking the place of peat moss, and for good reason. If you thin of a coconut, coir is the hairy stuff on the outside. It is a byproduct that comes out of the processing of coconuts. Think of coconut milk, shredded coconut, powdered coconut, coconut candies, etc. All of those husks used to just get tossed aside unused to pile up somewhere.
After processing, coconut coir behaves very similar to peat moss. I purchase large bricks of it off of the internet. I demonstrate this in the youtube video at the bottom of this post. When you add water to these bricks, they expand to about 10 times their size.
Unlike peat moss, coconut coir is pH neutral. It is neither acidic nor basic. This is almost always an advantage, but if you needed to you could always add a little big of vinegar to your water.
Other Alternatives
I haven't used any of these, but some other alternatives to peat moss are wood fiber, sawdust and composted bark. These tend to be acidic, like peat moss. It's important that they haven't touched the ground which would introduce soil contamination.
Aeration
A mix of 100% base material could be good enough for many plants. But to improve your germination rate, and decrease your risk of damping off, you want to add some aeration to your soil.
Damping off is a frustrating issue, where you seedling begin to sprout, but then the stems become weak and topple over ending in total failure. It is caused by fungal disease that thrives in moisture. We need this moisture to get our seeds going and help them develop roots, but at the same time we need to prevent damping off. Adding these materials will help with aeration to prevent damping off:
Vermiculite
Vermiculite are tiny rocks that contain a lot of air. It is super light to hold. If you look at the video at the bottom of this post, that bag might look like it would weigh at least 20 pounds. It probably weighs less than one.
Vermiculite is super aborbant, able to hold onto water 6 times it's weight. But at the same time, it aerates the soil, adding lots of air pockets to the base material it is mixed with.
Vermiculite is a rock, a mined substance. It is therefore not renewable. However, there is no scarcity of it, so this is not a concern.
Perlite
Perlite is like rock popcorn. It looks like styrofoam. It is a type of volcanic glass. It has similar properties to vermiculite, but is a little more dense. If you look at a commercial soil and see little balls that look like styrofoam, they are almost always perlite.
Like vermiculite, perlite helps keep the soil loose and airy. The big difference between vermiculite and perlite is that perlite does not retain water. So if you choose to use perlite in your potting mix, you will have to check the moisture levels more often.
Alternatives
Vermiculite and perlite are not the only ways to add air to soil. Some alternatives are: rice husks, pumice, polymer clay balls, horticultural grit, granite gravel and bark.
I haven't tried most of these myself, so I can't endorse them. Perlite and vermiculite are easy to come by, and a single bag goes a long way, so they are very convenient. I will often add grit to my mix when I am growing desert plants like dudleya or cacti.
I've seen polymer clay balls used in hydroponics. The idea of those is to give the roots of the plant something to grab onto, so they can spread in a healthy way. At the same time to the water and nutrient solution is able to flow through freely without any absorption aside from the plant roots.
Mycorrhizae
In the wild, seeds are not subjected to sterile soil. The soil is loaded with all sorts of microorganisms. Some of these would attack the seedling as it sprouts, others that provide benefits and actually protect against the bad organisms.
These beneficial fungi are called "Mycorrhizae", don't ask me to pronounce it.
The idea of adding it to your sterile mix is that is should prevent any bad fungi and bacteria from having open season if they get into your soil somehow. I know it is a common practice to use it in the nursery industry, but it is not a necessary booster.
This is the potting soil I use everytime. It's simple to make, easy to use, inexpensive and works consistently.