To save a picture with the maximum possible infoemation, do not resize it.
The original resolution of your picture will depend on a number of factors....The resolution supplied by the camera, the conversion process (if any is used), how the file has been processed and/or saved previously.
To set 300dpi (the standard for printing), go to image size, uncheck the box for resample image and set the resolution. This will not change the size of the picture in terms of pixels but will change the printing size.
If you change the resolution while the resample box is checked, PS will resample and, if needed, create new pixels by interpolating information which can result in a loss of quality.
Whether or not you need to change the resolution, by saving the file as a tif, you do retain the maximum information in a flat file.
hth
Norman