AUCTeX provides Font-Specifiers that one can use such for environments as described in the question and which are used often. For instance the following code

(add-to-list 'LaTeX-font-list '(?\C-q "\\underline{" "}")) 

will add underline to the list of font specifiers. This code must be called after LaTeX-font-list is defined. Then when you press C-c C-f C-q without selected region \underline{} will be created and the cursor is put between the braces. If you have a marked region then the result will be \underline{region}

AUCTeX provides Font-Specifiers that one can use for such environments as described in the question and which are used often. For instance the following code

(add-to-list 'LaTeX-font-list '(?\C-q "\\underline{" "}")) 

will add underline to the list of font specifiers. This code must be called after LaTeX-font-list is defined. Then when you press C-c C-f C-q without selected region \underline{} will be created and the cursor is put between the braces. If you have a marked region then the result will be \underline{region}