I'm also new to VPS and have learned a lot from support and the WIKI. The WIKI is very good. Current and accurate. The only trouble I have is sometimes it takes a few attempts to get the search phrase worded the right way. One thing I like about DH is that they don't force you in to creating several User accounts for different things, but you can if you want for extra securty.
To access the server, you will need to know the FTP host, User ID and Password, and a client that supports SFTP/SSH, like FileZilla. Also, the user account you use will have to have shell access. Log into the Control Panel and go into Users Management. Next to the user name, you will see if shell access is there. If not, go into Edit mode to enable it. Also, find the server name in the same row as the user name, under the columcn heading "Machine". This is the prefix for your FTP host URL. This is mine (actual numbers replaced:
I got the prefix from my users page, not sure where I got the rest. Keep in mind though it may not be the same for everyone. But that URL along with my user name and password and using the SFTP/SSH protocol, I got into the server, landing me in the "/home/my-user-name/" directory. This directory hold the site folders, and you can navigate up from there if needed.
For database and PHPMyAdmin, go to that page in the CP. Here you will find the database server URL and which user(s) is/are assigned to the database you want to administer. My server URL is "msql.xxxxx.dreamhosters.com". Go to that URL i the browser and login with the User credentials assigned to that DB. (you can assign existing users to existing databases or new databases. or create new users for existing and new databases).
I don't know about your other database question. Most of my DB experience has been with MSSQL, so I'm still learning MySQL administration. But, in the DH CP, I did find it very easy to create new and duplicate DBs, and backup and restore to and from local files. When I do go in to modify a DB, I will create a backup, and a duplicate. So if something goes wrong, I simply delete the original, make a duplicate of the duplicate giving it the original name and assigning the original user. This does depend on the site traffic - if it has constant use, I would need to take it offline for maintenance or temporarity disable certain features like registration.