Hi,
You can achieve this in two ways: first you can set an html "title" attribute from template or from javascript:
document.getElementById('fieldId').title = 'Tooltip title';
or jquery:
$('#fieldID).attr('title', 'Tooltip title');
The second solution consist in integrating a tooltip framework (eg. http://www.opentip.org/).
Also, if you use a Bootstrap based style project, you can use tooltip capabilities embeded. In this case, you can simply call it with jquery
$('#fieldID').tooltip(options....) (see more on http://getbootstrap.com/javascript/#tooltips )
Good luck