I don’t know what composer is, and I just want to download Slim to study it on my Apache. I can’t find a download link on your site 
There are links on github and, from your second post, I think your find those.
Composer is a package manager. And if you are a complete beginner I won’t recommend using a micro framework like Slim without basic knowledges on what composer is or git or what a package is and so on.
If you are not a programmer maybe go for a CMS. It may be easier to start.
If you want to learn programming on a PHP framework specifically here too you have ton of choices with different paradigms and learning curves, you can check out:
Zend, Yii (https://www.yiiframework.com/doc/guide/2.0/en), Phalcon, CakePHP, CodeIgniter or if you have several months ahead for good practice and learning you could start with Symfony or Laravel.
Check how they are built using composer and the many libraries of the great php ecosystem (e.g https://thephpleague.com/)
For either of these projects prepare for a serious investment and a great challenge in order to master the tool. Once you have seen some of the packages other projects use and how they are used and why, maybe you’ll start to be able to select packages on your own (http://packagist.org/) to build an app on top of a microframework like Slim.
If you’re new to the field, getting started with a micro framework like Slim might be slightly overwhelming if you’re unfamiliar with basic stuff like what Composer is or working in a Git environment. Composer is a package manager that helps manage your dependencies for PHP projects and is needed to use modern frameworks. If you don’t really want to program, you may be better off starting with a CMS because that will give you a simpler start to web development. If you are still determined to learn about a PHP framework, there are many options with varying complexity including Zend, Yii, Phalcon, CakePHP, and CodeIgniter. For a more in-depth, complex experience, Symfony or Laravel may be good options assuming you are prepared to really invest the time and energy to learn them. Regardless of the framework, if you learn how to work with Composer and libraries in PHP, you will be much better equipped to build and use more complex applications. Just know that, if working with tools like Composer and PHP libraries are all new to you, you can anticipate a steep, but exciting learning curve while you figure out your development tools and start choosing packages for your projects.