Can putting braces affect a lisp ?

Possibly: If the reason for the lisp is malocclusion of the teeth which in turn affects tongue placement as you talk, then correcting the alignment with braces may be useful. I'd invest in a few sessions with a speech pathologist to discuss tongue and mouth exercises you can do to diminish the lisp.

If you have a severe: Overbite or underbite, braces may help. If you have a frontal or interdental lisp from tongue thrusting, there is an appliance that can help reposition the tongue.