Using WordPress In-Built Customizer - This is probably the fastest and easiest way to create small but amazing changes to your blog or website. It's part of the WordPress default package and can create new tweaks to any WordPress theme out there. But the degree of WP customization you can get with this tool totally depends on the Theme you chose. There are some themes that don't allow customizations much at all. You can use these default themes as a base for creating your own tweaks and customize them with various plugins.
But when your theme comes with too many plugins it may not be possible to make any tweaks without these extra plug-ins. The best way to go about doing this would be to use WordPress theme generators to generate your customized theme. With a theme generator, you can have a glimpse of how a fully customized theme looks like before implementing all those fancy widgets and colors. If you're feeling lost along the way and don't quite know what you want to do with certain parts of your theme you can always seek help from the forums and other users on WordPress. You will surely come across people who can offer great advice on WP customization.
Other methods of WP customization include using plugins like Modou, SEO skins, and the built-in admin dashboard. Some of these things may seem more complicated than others, but when you really think about it they're very easy to implement and customize. You just need to be familiar with the most popular methods and then start creating your own awesome user-profiles and frontends.