Are you sure you're doing the long tail cast on? Loopy, loose and extra yarn from the caston is usually when you do the backward loop CO, which is only the thumb part of longtail cast on. But yes, tension does improve with practice. Give yourself a coupla days...
eta - Or you mean you have a big loop at the end of the first row? That's not unusual, and it'll get better after a while.
sue- knitting heretic