To start planting your seeds indoors you need to choose containers for the purpose. You can buy new flats, recycle containers from last year, or use any 3 inch deep tray.
If you are using last year's trays, they should be thoroughly cleaned, then rinsed in bleach water, to prevent your new plants from getting any diseases which may still be present from last year.
Fill the pots with a mixture of peat moss and vermiculite or perlite. You can buy this premixed, or mix your own. Peat is a lightweight sterile potting media which is excellent for starting seeds.
Regular garden soil should not be used as it can result in damping off disease in young plants.