Theme is basically design and functionality of our website. It governs a lot of features and updates. Broadly there are two types of themes: free and premium. Premium themes have more attractive and intuitive appearance. Also their functionality includes more features.
For wordpress bloggers, following is some basic information on themes which I explored:
Appearance > Customize: To change the customizing as per your need. (Not there in all themes)
Appearance > Header: To change the header image
Different themes have different options.
Theme > Theme options > See docs— to study about how the theme works in detail. Usually not there in free themes.
The widgets which appear on blogs are governed from: Appearance > Widgets
One of a good site to download free theme is wpexplorer.com
Basic things to watch in theme:
WordPress compatible (Theme should be compatible with wordpress)
Mobile compatible (If you access your blog from mobile, no distortions happen)
Plugins compatibility (You should check what all plugins would be compatible to your theme)
Layout (2 columns, 3 columns or other)
Review (Usually you can find the ratings and reviews of all wordpress themes)
Process to use theme:
First download the theme.
Now go to install theme > Upload > Choose file > Install now > Activate
The layout, designing and structure which you see in blogs is usually governed from menu.
Dashboard > Appearance > Menu > Create Menu > Manage Location > Select Main menu
All the hierarchies can be governed from menus and submenus.
Usually for more intuitive and attractive appeal, premium themes are being purchased. They can cost roughly from 30 USD to 100 USD and even more.
To buy premium themes, visit: Themeforest.net
Usually, you can have a live preview. Then purchase the theme. After payment, you will receive an email which will have link to enable you to download that theme. Then download the theme. Double click (To unzip) on your downloaded zip file. It will have help file also. Make sure that while uploading, you do not upload this help file. Only upload wp.zipfile. Then theme > Install > themes > Upload > Choose file > Install > Activate.
Few of the good premium themes which I came across include The thinker, New road, Marlene and Lifestyle Pro. You can also find videos of all these themes on youtube to get more idea on how catchy they look and what could be advantages to have them.
Another good theme provider is Mojothemes.
Use premium theme for advanced usages, high level fame and e-commerce purposes.
This blog has normal free theme of 2016(Twenty Sixteen). For normal blogs and small audiences without any payment transactions, free theme is good enough.
Practice gives you a lot more help. The more you explore customize in appearance of your theme, the more you would be able to use wordpress platform to your advantage. There are lots of thing which you can do. Just get your hands dirty and start exploring the features one by one.