The list of user privileges and the requirements to reach certain user levels are listed here:
Any privileges not listed in the link above are reserved for administrators (ie deleting someone else’s topic).
Trust level 0 (New): This is, AFAIK, intended to discourage bots. The requirements to advance to TL 1 (Basic) can easily be completed within half an hour of browsing. Until then you can expect major limitations (ie no private messages).
Trust level 1-3: You can advance through these levels by engaging on the forum (reading, writing, getting/giving likes) as long as you don’t get suspended or silenced.
Trust level 3 (Regular): Unlike all the other levels, you can lose TL 3. As the name implies, you have to be a “Regular” user of the forum and if you don’t visit regularly you will get demoted.
Trust level 4 (Leader): This can only be given by staff/administrators.