First of all... I've been crocheting for 20+ years and still have trouble with getting straight edges. For that reason I will usually pick a pattern that is more forgiving (like one using a huge hook) if I want pseudo-straight edges. Or do something in the round (I do great with granny squares, maybe you can try a granny square afghan?).
As for the tension, it's a matter of how you hold the working yarn in your left hand. I actually hold the yarn exactly the same for crochet and knitting, which happens to be the way demonstrated in the videos here (around the pinkie then over the index finger). I've heard of people threading a bead on their yarn and working on keeping that bead between the pinkie and the palm as a way to work on tension. It is a matter of practice.
Another think you can do to work on the odd shaping, is to chain a small number, like for a washcloth. Work the next row and before you turn count the stitches. Work the next row and before you turn count the stitches. If your number is changing each row, rip it out and figure out why, keep trying until you can make a square dishcloth. It's a little easier on the smaller scale.
Oh, and I'll also point out that single-triple-single-triple will not give you a straight line at any point, your triples will curl because the singles don't have the height they need if you try to pull it straight, or the base chain will go all wavy. The only way I can think of that that'd work is if each succeeding row puts a single in the top of a triple and a triple in the top of the single The rows still won't look straight though, you'd have a bit of a zig-zag pattern going.