You need to assemble the copy (text) and images, agree the size of the poster and number of copies to be printed and who is to print it (and who's going to pay for all of this!).
You also ned to have InDesign (which version?) and know how to use it.
You probably need some training. There are lots of books and there's online tutorials such as those from Lynda.com, as follows (I believe you can get a week's free access).
And this one is specifically about poster design: Watch the Online Video Course Designing a Poster
And you can ask specific questions on this forum.