How to keep high DPI in pictures when using Photoshop?

What causes DPI in pictures to go down when editing pics in photoshop? using filters or ';contrast';? resizing pics? if i turn up the ';contrast'; on a pic from 0 to 100, does that change the quality of the pic or DPI? how can I be sure to keep the quality of the pic in photoshop? is there a certain way to export the pic?

Also, is there any way to check and see what the actual DPI is in photoshop?How to keep high DPI in pictures when using Photoshop?
DPI is Dots Per Inch(squared). The only thing that chages the DPI is if you change it in the image or canvas properties. Resizing the image can also change DPI. Nothing else in photoshop will do that.How to keep high DPI in pictures when using Photoshop?
Go to the image pull down menu. select image size that's where you adjust the image size. Generally you can reduce the dimensions of a picture and up the dpi but not visa versa.