I installed SuiteCRM Version 8.20 successfully.
After login I only see a screen with some dashlets (accounts, leads etc.). But there is no possibility to enter any new item like an account.
In former SuiteCRM versions(e.g. version 7) there was a menu for selection of item lists.
How can I start the menu in version 8? Or any other possibility?
I think you’re pointing to the wrong URL, or you have a wrong ModRewrite configuration, or .htaccess issues.
See the live demo to see what SuiteCRM 8 should look like with the full navigation, top menu, etc.
“wrong url” may be.
In version 7 you could start the file “install.php” from the root of the installation directory. In V.8, however, the file is located in the subdirectory "public/legacy. From there I started the installation. This also starts the call of SuiteCRM from this directory.
What am I doing wrong?
You’re not following the Installation instructions in the Documentation 
You not supposed to point to install.php; you should just be able to point to the base URL (like www.mysite.com or 10.0.0.1 or whatever) and the installer would kick in, if the product is not already installed.
If that fails you need to work on the web server config, maybe it’s ModRewrite, maybe it’s an .htaccess issue, or just a wrong base dir in the Apache conf file.