From the end of your first lesson you will be better equipped to defend yourself. At the end of your first class you will know how to perform several punches and kicks correctly as well as how to block and evade your attacker.
At each following class you will build on this, getting better every class you attend. Your confidence will also grow with your new found skills which will also benefit in keeping you out of trouble in the first place.
Most students would agree that "you learn Karate so you'll never need use it", and for most of us, this will hopefully apply in the sense of hopefully never having to defend ourselves or our loved ones in a life-threatening situation. However, with Karate training, you will have a much better chance of doing so starting with your first training session.
Students find that they are much less physically threatened and are therefore able to calmly take verbal control of the situation before it escalates and gets out of control. I have a saying