How do I get to the web UI in CraftCMS to install it?
How exactly are you running Craft?
Ubuntu/Debian are just Linux operating systems, they don't necessarily come with a web server stack installed but it looks like what you're running is at least partially working.

If you getting the welcome screen, it sounds like Craft got installed so I wouldn't try reinstalling unless you can confirm there's no tables for Craft in your database.

If you can get to http://examplesite.com/index.php?p=admin/dashboard, that typically means the web server's rewrites aren't working and what you have now is a webserver configuration issue—not uncommon.


To solve a web server issue:

Apache: make sure .htaccess is located inside Craft's web folder and AllowOveride All is in your virtual host config.
Nginx: you need some special rewrite rules to get it going.
You may also need to set omitScriptNameInUrls to true in config/general.php.

If you can't get to the dashboard or the login screen, something else happened, perhaps a botched install and you should revisit how your web server got installed.