Can you be more specific of what you mean by "on screen"?
Things like the Player/distance or raid timers on screen?
If you just want something very basic and simple, use MCP->Timer. It's the most basic method and does everything for you based on your input.
If you want something specific, you are looking for HUD in the lavishwiki.
Note: Do NOT put any server calls in the HUD screen. These update very quickly forever until they are removed. I'm sure it tells you the default, but I think it's every couple of frames.