Typically, web based imaging is in 72 PPI (Pixels Per Inch) and is typically the default setting for Photoshop. (And GIMP)
When you create a new document, don't worry about the dimensions (Yours seem fine for typical print size), just bump up the PPI to 150. (There should be an option for this when you create a new photoshop document.)
After that, just create your image as normal, and make sure whatever printing site you're using accepts .PSD files (The photoshop format) or .PNG (For images without artifacts, as gif loses color quality, and jpg will create artifacts.)
And to ensure commercial (Or close to) printing, make sure the site offers at least 150 DPI. (This is just for higher quality prints, it doesn't affect your document if it is any higher, really. This is just a measure for printers. Anything lower will look fine at a distance, but if it's going to be hung up on a typical house wall for viewing, 150 and up is a safe bet to ensure no quality loss at close inspection.)
Other than that, photoshop should automatically calculate the resolution based on the PPI and whatever your document size is set as. (In your case, centimeters)
Hope this helps! I haven't worked with online printing companies, but this should be a good start to ensure a pretty nice looking poster.
