Prick out only the healthiest-looking seedlings in the seed tray and compost the rest
Seedlings from sowings into seed trays will soon become crowded, slowing their growth and making them vulnerable to disease, so prick them out into bigger pots once they’re large enough to be handled without damage. You’ll have much stronger, healthier plants by doing this. If you leave them in the seed tray for too long they run out of nutrients, go pale and just don’t have enough room to grow well.
Fill pots with compost suitable for the plant you are growing. Peat-free multipurpose compost is good for annuals and veg plants that are fast growing and will soon be planted out. Perennials should go into longer lasting and more nutritious potting compost with some added loam, which helps with moisture retention and provides extra nutrients.