How do I customize a template?

‘how long is a piece of string?’

some themes have more-or-less elaborate option pages in the dashboard;

otherwise by editing the theme files, such as style.css, index.css, …

http://codex.ww.wp.xz.cn/Editing_Files
http://codex.ww.wp.xz.cn/Using_Themes
http://codex.ww.wp.xz.cn/Theme_Development

and websearch for individual problems; searching the codex for more details; learning css, html, php (only a bit); trying, failing, asking; looking into other themes; …