wait until you have a 5th node starting on the plant. that's when it's officially not a seedling anymore. that's when you'd start with 1/4 strength. for vegetation, you want something high in nitrogen, so something like 24-2-2 would be great. if you can find that in a liquid, it will be easier to measure and easier to mess with, IMO. you can probably find something at a lowe's, home depot, or gardening store. walmart sucks