Congratulations. The fact that you have decided to give a lecture means that you have already won half the battle. As for the other half, most people are not as bothered by our stutter as we think them to be. I teach. Earlier I used to be very anxious about stuttering in front of my students. But I realized that they don't care about my stutter as much as I do and that helped a lot. So what I do now is in my first class I announce that I stutter and I tell them to interrupt me if they do not understand what I am saying. That takes off a lot of pressure. Also I use props, like the blackboard, and body gestures. Those help a lot too. Maybe you won't be able to learn how to use body gestures in a week but use props if you can. The exact words that you say are not very important, what is important is the message that you convey. And third practice your lecture as many times as you can. In the long run, joining a nearby toastmasters club would be very helpful in this regard.