Unless you want to make a bunch of design customizations to your WordPress theme after you install it (not ideal for the average user), it’s always best to select a theme that already has the major features you want. You should make a list of the main features that are most important to you and search for themes that meet as many requirements as possible. That way you aren’t spending time customizing the code and design later.
For example, how many columns do you want ? Do you want the ability to customize fonts?
If you choose to go with one of the free themes provided by WordPress, they can be a bit more challenging to customize because they may not give you as many customization options through the WordPress admin panel. If you have money to spend, you can purchase a premium theme, and it may give you more options for customization and provide you with support.
WordPress is very easy to setup if you use the theme “as is”, but making design customizations depends on how customizable your theme is and how much HTML and CSS you’re willing to learn. Major design customizations will require you to learn HTML, CSS and PHP, or may even force you to hire a WordPress Programmer.
If the primary goal of your site is to blog about your garden then choose a theme that is geared towards the goal of blogging, or if you are going to sell banner ads in the header, choose a theme that has available banner space.
The site navigation (menu) is one of the most important parts of a site. You may need a very simple navigation menu with a few options or you may need a more elaborate one.
Responsive themes automatically adjust and display favorably when viewed on mobile (smaller) devices. The easiest way to ensure your WordPress site is responsive or mobile friendly is to use a theme that is already coded to be responsive, like the twenty-twelve theme.
When you search for a free theme you can do it via WordPress admin panel. It allows you to sort by specific features you are looking for. When you go to Appearance >> Themes >> ADD NEW, you will find a filter (see below).
You can search for a paid theme here: http://themeforest.net/ or simply do a google search for “premium WordPress themes”.