Hi, yep this is the 'greeting' thread but to answer your question...
What I do to join a new end of yarn, is to loosely tie it to the old one, work a few rows then go back and rearrange the knot so it doesn't show and tie it tighter. But that may not work so well for garter stitch. There's a few methods for joining yarn shown on the Tips page.
sue- knitting heretic