Being able to modify a theme to suite your needs, it is essential to understand their basic elements: .info, .css and .php
If you can't see changes in your browser or if your site acts strangely, try to clear the cache first.
Install the module Backup and Migrate to back up your Drupal site. This is probably the easiest way to back up and restore your Drupal MySQL database.
To make the development and administration faster you can install the module "Administration menu" (drop-down menu).
If you encounter an error message or fatal error after updating/installing a module or theme: Try to "undo" first
If you have a problems or questions get help from the Drupal community: Ask questions on drupal.org or on famous howto sites.
The default error messages Drupal gives your users can be intimidating and not really useful. Thus I recommend you to configure specific error messages.
If you want to save time, I recommend you use Drupal Distributions, which make it possible to quickly set up a complex and use-specific site.
Views, PathAuto, References, Display Suite, Panels, Chaos tool suite (ctools), BUEditor and Backup and Migrate.
Installing Drupal on your local machine(s) offers you manifold advantages: It allows you to work faster and offers you the perfect playground to experiment.