You can plant seeds in almost any type of container, but using a commercial seed tray may cut down on the need to transplant up to a bigger pot. I have seen people start seeds in egg cartons, which, you know, are very shallow, and if you only start a few seeds each year egg cartons may work just fine for you. The new ones that have a clear cover work the best. The cover keeps the moisture in until the seedling come up.
Plastic Seed Trays
Black plastic seed trays come in kits with the sectioned wells, an open tray to hold the seed tray, and a clear plastic cover to retain moisture. The trays have small round or square wells for anywhere from 12 to 72 seeds.
Biodegradable Peat Pots
Another type of tray has biodegradable peat pots, called Jiffy pots in the tray kits instead of plastic wells. The peat pots eliminate the need to transplant your seedlings because you can separate the wells and plant the pot right along with the seedling.
In addition to peat pots, there are Jiffy pellets available. These are small round dehydrated and flattened out pellets that swell up to about 1 ½ inches and are covered with a fine cloth mesh which the roots can grow out through. These can also be planted directly into the garden.
Cow and Coir Pots
Other biodegradable pots include cow pots made from 100% odor-free composted cow manure and Coir pots made from pulverized coconut shells. The biodegradable pots fully decompose in the ground so no need to transplant and they add soil-conditioning natural nutrients to your garden.
Seed Starting Mix
When planting seeds, you will have the best results if you use a good, organic commercial seed starting mix. These mixes have nutrients in them, are light and airy to allow tender roots to grow, and retain moisture well.