300 DPI is the best resolution possible. As you move up in print size, the DPI will decrease. This is not a problem as you will naturally view your image from further away. The larger you would like to have your photos printed, the greater the number of pixels should be. While you can add pixels to your image to get 300 DPI, we normally do not recommend this as the results are not always predictable and files will become unnecessarily large.
Photos straight from your camera will have the best resolution, while photos coming from social media will have moderate to poor resolution. If you're photos are from social media we recommend small and medium sized prints. If you've shot your photos from a DSLR or even modern camera phone, you can print at maximum sizes with impressive results!
For the more technically inclined, below is an general overview estimating quality in relation to pixels: