If you evaded the first hit of a string, yes just block. Some strings don't track and you can continue to sidestep to get your opponent backturned (BT throws are inescapable) but until you know those strings just play it safe and guard.
When you evade a single hit move, like an elbow or something, you've typically put your opponent in forced nitaku. Most characters' 3K will cause a special crumple on side-turned opponents when it hits CH, you should check to see if your character gets that off of ST 3K. If you have a guard break, that becomes much better on ST - Lau, for example, actually get's a free 3P+K launcher off of his guardbreak on ST. If you have neither of these tools, you still have the advantage and should apply standard throw/mid mixups.
About strings - while you're playing, if you notice your opponent is using a lot of strings, see if any of them appear to be highs. Highs can be ducked under for huge rewards - at first, just try hitting 2P when you think a hit of a string is high.