Starting from seed is not always an easy task, especially if you're starting indoors. Here are some helpful tips:
Choose a growing medium. You can make your own with compost, perlite or vermiculite and coco coir. You can use a premade seed starting mix, or you can use a soilless medium like coco coir. When using a soilless medium, you'll need to fertilize your seedlings once they develop a set of true leaves since soilless mediums don't have the nutrients your seedling need to continue growing. Despite not having any nutrients, choosing a soilless option is beneficial because you avoid potential pests like gnats and eggs that may be living in seed starting mixes.
Make sure you choose the correct sized pot or seedling trays. Not all seeds should be growing in the same size pots or trays. Tomatoes grow fast and tall with vigorous roots. They should be planted in something deep enough for the roots to spread without getting root bound before it's time to transplant them. Squash and zucchini need lots of room. Their roots grow fast and the leaves are large. They need a deep, wide container to start in unless you plan to direct sow them.
Plant your seeds at the proper depth. This means that you should plant your seeds as deep as they are long. Tiny seeds like basil can be surface sown and lightly covered with your growing medium. Seeds like beans or squash should be planted about 1/4 inch deep.
Make sure your seedlings get enough light. If you're starting indoors, a sunny window will not suffice. Because strong light is only coming in from one direction, your seedlings will lean and stretch towards the window creating leggy, weak seedlings. You should provide your seedlings with overhead lighting by using grow lights, at least 2-3 inches above. As they grow taller, you can raise the light source. If this is not an option, if it's warm enough outside, you can put them outside during the day and bring them in at night. If it's cold out, a clear plastic tote can act like a green house, keeping them warm during the day. Then you can bring the tote in at night. Leggy seedlings aren't an easy fix. You can plant them deeper, but you risk stem rot. However, something like tomatoes plants that grow a root system along the stem can be easily fixed by planting them deeper.
(These are leggy seedlings that did not get proper lighting. The stems are thin, weak and long. These are not healthy plants)
Keep your seedlings warm. To avoid damping off, which is a disease (fungus or mold) that thrives in cold and wet conditions that causes seedlings to wilt or look as though someone pinched them at the stem, keep your seedlings nice and cozy warm.
Bottom watering is best. When you bottom water, you encourage seedling roots to grow down and take in water via the capillary method. It also helps develop a stronger root system, and avoids attracting gnats.
Fertilizing your seedlings will help them have a healthy start. Even if you are using a seed starting mix, you should still fertilize. It's best to wait until at least one set of true leaves form, then fertilize using a liquid fertilizer (fish, kelp, or a mix of the two is a great option) at half strength.
Don't overthink. Do the best you can, and remember that garden mistakes are an opportunity to learn what to do or what not to do the next time.