Having the main points on a piece of paper, overhead, or powerpoint presentation is important, as is going through the talk first by yourself to make sure the timing is roughly right.
When you are giving the talk/presentation and you can see that you are going go overtime, don't rush, stay calm and just leave parts of your talk out. Remember no one knows what you are going to say, so they will be none the wiser. Also if you leave parts out you remain in control, rather than being pressured to finish quickly.
You also may be very nervous, but that is generally not visible to other people, or they will be understanding.
You will get better at it over time. I know I stuffed up my first presentation really well.
The main thing is to be well prepared, you will still be nervous but that is to be expected.
Cheers