The first two lines
There is a succinct and useful post at the addictive FemaleScienceProfessor blog entitled "The First Two Lines", in which FSP advises a student on how to give a good presentation.
"When my students are preparing presentations for conferences, I always tell them that the first 2-3 lines are particularly critical and I want them to think carefully about how they will introduce the talk. After the first few lines, the rest of a talk is typically straightforward (data, interpretation, conclusions), but the first few lines are where you either grab the audience or you don't. This is when you lay out why the work is interesting and important, and why anyone should care about the rest of what you have to say. In fact, it's a lot like writing a proposal."
Read the rest of the advice here. If you are inexperienced or unsure about giving a scientific presentation, or indeed writing a paper, I think you'll find it very helpful. Certainly the student who received the advice found it so, according to the comments to FSP's blog post.
