The theme on your website is the backbone of the site, it is where the code for the layout, design and fonts/graphics is housed. WordPress has many thousands of free themes available to download to use on your site. It is important that you take your time to choose the right theme for your site.
First off, you need to be aware of the purpose of your website. Of course, we all want a website to get as many visitors as possible, and hopefully convert those visitors into sales! Not all websites are there to perform the same function; some are purely there for information about products and services, others sell items through ecommerce facitilities and others have booking availability etc available. If your theme does not match up with the core purpose of your site, it will not work as well.
The theme needs to be clean, minimal and neat looking to have the best effect on your site. Some themes are packed full of over the top effects and gadgets which are not necessary to the functions of the site, instead they just add extra code and slow the site down.