Well first off, you need a printer that can already print poster sizes. Your file should have a ratio for a standard poster of 2:3(4x6 shape), 5:7(5x7 shape), 4:5 (8x10 shape), or 11:14 (11x14 shape). The safest DPI I say is 1200 especially if the Image size is coming up 2 inches by 3 inches or something like that. Though it really should be 300+ DPI regardless. http://www.theposterplace.com/Sizing.asp is a good reference.
Make any necessary adjustments you need to do for that specific printer then let it go.
It's really expensive just to do one. I did one at my school and it was about $40 to print.
go to file, then click print, then click print again
yes there are you can print them at codelyoko.com
With a printer
Print Runner, UPrinting, Vista Print, Make Signs, All Posters, Poster Brain, and Zazzle are all places an individual can go in order to design and make cheap posters for both advertisement and personal decor.
You can get them from the Oregon state website. The posters are available to print out here http://egov.oregon.gov/BOLI/CRD/C_Postings.shtml.
You can download Photoshop, GIMP, Photoplus, or you can use the online website Pixlr. All of these EXCEPT for Photoshop are free. I just started to make posters and I use Pixlr.
With a Printer.
Posters, Magnets, Banners, and signs.
print posters and campaign materials
The best place to shop for posters and prints is at Vista Print. This company allows you to buy these items at a cheap price and the quality of the posters are very good.
The federal government required certain business post posters in public places for employees to see. The Department of Labor posts these posters and their rules on its' website.
There are several companies that offer custom posters. You can try Vista Print or Boise Custom Poster. You can go to various printers for ideas for fun custom posters.