If you don't have someone to teach you how to snatch, I wouldn't try and do any snatches. It's no easier to keep your arms straight in a snatch than it is in a clean. And, because the movements are different, you can learn one correctly and then have problems in the other.
First, start by internally rotating your shoulders and then pushing your chest up (think a lat spread pose). And stay over the bar longer. You are engaging your back to early. Try and think of pushing the floor away, or like you are unfolding yourself in a straight line.