Tell them what your going to tell them. Tell them what you've told them. It is crucial that you start your talk by telling people in summary what it's going to be about. This kind of sentence sets you up. The audience are given a sense from the word go that you mean business and know what you're talking about.
Many people find it necessary to describe the structure of the talk.
I suppose it's not a bad idea but let's face it, if you tell a good story you don't need to give away the plot before you even start. So you might consider skipping this. This is where you put it all into context. One sentence at a time you lead your audience into the talk. Two historical sentences, a discussion of why this area has become important, where your work fits into the picture referring to other leading lights of the field, other work that people in your group have done, and then you are ready for The Body of the Talk.