Can I grow plants on the window sill?

You can........but not the best situation and here is why!

1. Unless you have a 'sunroom' you only have strong sunlight on a window sill for a period of time. (if at all)
2. Northern facing windows have even less sun strength.
3. Above photo shows what happens when plants have to stretch to reach for the light called "etoliation."
4. Yes, if you turn them, they will stretch the other way (that doesn't mean it's okay)
5. Long and lanky plants do not do as well when transplanting, and are best 'trench' planted.
6. Provide florescent lighting that should be 4" above the plant tops, so you may have to lower or raise the lights or the plants to maintain the 4" goal.

Best plants are short, stocky and young. Tomatoes best when 6-8 weeks old before going to the garden, 8 weeks old for peppers. Tomato plants benefit from being transplanted into graduated size pots 3 times before going to the garden.

1. start in a seed flat or similar size container - tiny about 1-2" sq. inch of soil per seed
2. when 2-3" tall, transplant to a 6 pack size container, about 4-6" sq. inch of soil per plant
3. final pot -  a  2-4" square pot size

So about 2-2.5 weeks per pot size. If you only want to transplant twice, than go from seed flat to a large or deep 6 pack size container. Every time you change sizes, sink the plant all the way to the bottom of the container, and fill in soil around the stem. Best if plant is buried up to the true leaves.
You can bury the 'seedling' leaves - they would be the 2 leaves lowest on the stem. Questions? Please post them in the comment box. It's not too late to start either one, they produce till frost!