Toward and towards are both correct and interchangeable: you can use either one because they mean the same thing.
Many sources say the s-form is more common in Britain than in the United States. The safest choice is to consider your audience:
- If your audience is primarily American, use toward.
- If your audience is primarily British, use towards.
For a deeper analysis of the usage patterns of toward vs. towards, see Jonathon Owen's column, "Towards a Fuller Understanding of Usage," which argues that "towards is seemingly rare in American English because copy editors make it rare."