To install ImpressPages CMS you need to have access to your MySQL database. When creating a such database you provide the name so you should know it. No one else does. Or if your hosting provider gives you online one database, you should know it, too. In some hostings database names comes with prefix, like hostinguser_dbname. Check that out.
To access the database you also create a username and password (for database, not installation, don't mix that up). It means, you should know it if you created it, or hosting provider must give you those. Very rarely username is "admin" and password so simple. As for DB names, usernames for DB also may come with prefix, like hostinguser_dbuser. Don't ever tell anyone those passwords. Even to us. You are compromising your security.
Prefix doesn't matter if you have only one ImpressPages CMS installation on the same database. Keep it default value.
To sum up, only you should know the values of required fields. It comes from your hosting:
1. Host - by default it's "localhost", if not, your hosting provider should tell you exactly what it is then
2. DB user - ask your hosting provider
3. DB user password - ask your hosting provider
4. DB name - ask your hosting provider
5. Prefix - keep default or make sure you now what you do if you want to change