Clairdo, you can feed bedding plants with a general fertilizer, starting 1/4 strength if they are young plants and building up to full strength by the summer. But it would help to know which plants you have. There was a good thread on the Far North Forum about which plants don't like or don't need fertilizer. I will post a link to it below.

Tomatoes are best fertilized with a specially-formulated tomato fertilizer, such as Miracle Gro for Tomatoes. Again, use it very diluted while they are young.

I hope that helps.

Here is a link that might be useful: Plants not to Fertilize