If you can't make what you need by hand, you can create a smaller version using Word or PowerPoint or Publisher and then simply take the file to a printer/copy center (like Kinkos). Note-you'll want to find out from them what specifications and file settings to use.
Prices vary and quickly escalate if you are using color.
But before you expend those funds, you might want to check for other solutions:
1) Does the school/district have poster printers that might be used?
2) Does one of your parents work at a company with such printers and could they ask their company to donate the printing?
3) Do you have a supportive company that might help? Now, even relatively small organizations often have graphics/publications staff. The might be willing to help you create and print the poster.
Also - on creating it by hand....
I'm pretty good with a computer, but am artistically challenged (to say the least!).
When I have to do posters, I create segments of text and clip art and then print them on brightly color paper. I cut the elements of the poster from this and then just arrange them and glue onto the poster board. That can work out to be an attractive, easily read sign.