Congratulations on your entryway into seed sowing! Growing from seed can be a rewarding experience and one that demands patience. The information below can likely be used for a variety of tropical plants but is primarily intended for Begonia seeds.
There are a variety of mediums that can work well for germinating, listed below. All mediums should be provided in a depth of at least 2″.
Peat– my favorite substrate for starting seeds, I have used unsterilized peat successfully, but think the seeds are better off using a sterilized peat that is microwaved or has had boiling water run over it. Make sure to pre-saturate the peat before putting into the germination container as it takes some kneading to get the right consitency. Peat its hydrophobic so make sure to work it until you get an evenly moist substrate. Spread peat evenly and pat down very lightly to create an even surface.
Sphagnum– works very well for a variety of plant groups. It should be finely chopped by hand or in a food processor/blender to ensure even spread. This should also be sterilized, mostly to prevent the sphagnum itself from coming to life (can outcompete small seedlings). Sphagnum should be soaked for at least a day prior to use to ensure it saturates. After it’s fully saturated, squeeze most of the water out until it stops dripping. Pat down sphagnum very gently in the germination container. You want to close up any possible holes that seeds could leak down into.
Perlite– the smaller grade (1 and 2) work best for germinating, as the larger grades allow seeds to drop in crevices where they might not get enough light. Once plants have put out some more mature leaves they should be fertilized to prevent a nutrient crash. I do not think perlite needs to be sterilized prior to use.
Perlite & Vermiculite- mix evenly and let soak for an hour or so prior to use. Spread evenly. I don’t think either of these need to be sterilized.
Aim for temperatures around 75F. Ideally, nighttime temps don’t drop below 70F or so. I have germinated seeds with nighttime lows of 65F in the winter, but, it seems to slow them down and in some cases it might prevent them from germinating. You can safely get up to 80F, but anything above 85F should warrant caution.
The intensity of light seems to be less significant than the duration. I use 1200lumen LED lights at a distance of about 12″.
Water & Humidity
Pre-moisted any growing medium you wish to use. It should be evenly damp, but not wet. The medium should not be allowed to dry out. Maintaining high humidity is important, and seeds should be kept in an enclosed space (ideally a small one) that keeps humidity 90-100%. Watering or misting the seeds before they have germinated is undesirable as it may wash the small seeds into the substrate, preventing light from reaching them and making germination impossible.
Begonia seed should be light dust on the substrate’s surface. Do not plant the seed, or attempt to push into the substrate. Additionally, do not water or mist following sowing as the seeds may fall into the substrate. A small deli cup size is sufficient for 20-50 seeds. It is best to sow many seeds as the abundance of seedlings increases humidity and creates a more desirable growing environment.
The time it takes for seeds to germinate varies greatly. Begonia cathayana, for example, germinates in around 10 days, while versicolor takes 4-5 weeks. Some species take longer. You will likely not see growth from any before day 5, and most likely not for a bit longer. Only after 8 weeks should without germination should the effort be deemed unsuccessful.