The look and feel of your Jooma site is controlled by your template.  And, there are many templates available for your site.

In addition, non-Joomla templates can be converted into Joomla templates (but it is not a quick and easy process for the beginner).

So, I generally recommend that you (especially beginners) begin with a pre-designed template, and then customize it to their graphics, images, colors, fonts, etc. rather than by starting from nothing.

But what if you really do want to build your own template, or at least understand the process?  The good folks at JoomlaShack have produced a nice, step by step tutorial about the process.  You might find that to be useful reading.

Click here for the article.