Hi
@SamShine77
It is unclear if you have all the specifications to guide you down every step so this is more generic at this point
1) You need to know the size limits in both width and height for the print job (in cm is just fine)
2) You need to know the desired resolution in pixels per inch or pixels per cm (300ppi is photographic quality and other types of print typicall don't go below 150ppi
3) Your project first must be cropped to the required aspect ratio (this is optional if allowed by the onlineprinter.
4) Use Image > Image resize setting the sizes in cm to the desired result. You also add the desired PPI or pixels per cm. Make sure the resample box is checked.
5) Save the file with File > Save As (or Save As Copy) to JPEG using a high quality setting (10 to 12)
The above steps should give you your output with the correct, cm size with the correct resolution and ready to go. You then upload that file to your onlineprinter.
Hope this helps
Having more details from you a forum member could walk through the steps more specifically and there are many here that could help
John Wheeler