These words are interchangeable. Using either is grammatically correct. The "s" doesn't change anything although "toward" tends to be used more often in American English whereas "towards" is more often seen with British English. Personally, I choose which I use based on how the word sounds in the sentence.

Give me an example, as I don't know any exact rule..... sometimes towards can be past tense and toward now. Moving toward him........He moved towards me.