How can I increase my digital photo resolution to 300dpi?

Hi,
I have some a photo taken with my digicam and it unfortunately has 180dpi.
I want to make a poster about 11 x 17 inches or larger. But when I start a new document and place the photo, it is so small…I think it’s because of the 180dpi…
How can I increase the resolution of it to 300dpi so it shows bigger with a still good quality?
I know that I can’t just resize the image because then it will become grainy and poor quality.
Thank you for any help 🙂