You did not say the dimension of your poster but there are two possibilities. The first is to photograph the poster. I have a tripod and a ball mount that would be ideal for this situation. Not having that you could tack or tape on the wall. You can use your photo software to straighten, crop and adjust the image and print any size you want.
You could also take to a printing store such as a Kinkos where they have large format scanners, have them make a scanned digital copy to a disc or thumb drive. That would likely be easier than photographing.
I think you are asking how to make something that is as large as a poster. If you use Photoshop or another digital scrapping program, try creating a canvas that is the size you want. I just went into Photoshop to see and I made one that was 30x50 inches. Then, you put your images, etc, on that. You will definitely have to have it printed somewhere, whether Kinkos or another printing place. You might find that printer first and ask what sizes they can print.
You could create your image first, then zoom out and divide the picture into sections. Print each section separately and tape them together. You would probably need software that would allow you to do that.