You can begin using Semantic by enabling Semantic mode, a global minor mode: type M-x semantic-mode, or open the ‘Tools’ menu and click on the menu item named ‘Source Code Parsers (Semantic)’. See Semantic mode.
When Semantic mode is turned on, Emacs automatically parses each file you visit. You can then use Semantic user commands in those buffers (see Semantic mode user commands). You can also choose to enable a number of “helper” minor modes for saving tags, displaying tag information, and so forth.
To enable Semantic mode each time you start Emacs, add the line
(semantic-mode 1) to your initialization file. See Init File.