Fertilizing?

It's been about 2 1/2 weeks since I began growing, the third set of nodes are starting to grow and I'm not sure about when I should fertilize next. I don't want to burn or, even worse, kill my plants because of too much nutrients. So can someone give me some advice about this issue? It's my first grow and they're looking gr8 so far!

I would show you what they look like right now but it won't let me upload the photos in an attachment, oh well!