The user titles specify the user group that one is in based on their activity, posts, likes, and etc. Momo's Minion (Registered) is the first user group people are in when they register (and have verified their e-mail), the step up would be Teto's Territory (Registered+), and after that would be Ruko's Ruffians (Active) I believe.
The final group members can achieve through just activity is the Defender of Defoko flag (Promoted) that opens up the ability to change your text colour in the
chatbox and gain additional permissions. To change your user title to something unique you can buy the permission (the permission is temporary but your user title is permanent until you change it again) in the
store. I was able to achieve getting this flag without posting on the forum at all and only sticking to profile comments, logging in, or being followed by others.
You can get the Supporter flag by donating to show your support for UtaForum! It doesn't add much permissions, but it does give you the ability to edit your posts in the chatbox and gives you a glittery green username.
You can read more about the user groups here:
https://utaforum.net/help/permissions/
Edit: To add on to my message, I think it's worthwhile to note that you can keep track of your UtaBucks earnings through this link:
https://utaforum.net/credits/transactions
Don't abuse certain ways to gain UtaBucks though! It's against the
terms (Do not attempt to game or manipulate forum mechanics) you agree to when you register on the site.