The Joomla basic install comes as just that - basic. However, there are thousands of extensions that allow you to easily add functions to your website. If you need to add a calendar to your site, or a map, or a shopping cart, there are extensions available to do all of those things and more.
Extensions are typically easy to download and install. Some can at times be a little complicated to configure so they work properly on your site, and display just the way you want. However, usually each developer has a forum for his extension so you can find answers to your questions and even get individual support. This is very important for extensions. Do not add an extension to your site for which you can not get support. Extensions may impact other things on your site and how they work. Installing one extension may cause another extension to quit working properly. If you install an extension that breaks something on your site, you can't figure it out, and you can't get support it can be very frustrating. Which brings me to recommend one of my favorite extensions.
One of the first extensions you should install is Akeeba Backup. This is an excellent free extension that will allow you to backup and restore your site in the event that you loaded an extension that messed everything up and you can't figure out how to fix it.
I have my favorite extensions that I use repeatedly. Many of my favorites have become so because their developers provide excellent support. I would be happy to recommend a handful of extensions that I have grown to love that will make your life easier, and if you ask, I would also tell you about a few that I would not use. I will happily pay a few dollars for an extension that is well supported over a free extension that is not.
There are several websites that provide reviews and ratings on Joomla extensions. There are two that I visit frequently. They are very helpful when determining which extension is best for you.