I would say "I'm at work right now".

Other examples:

I'm at home

I'm in bed

I'm in the kitchen

I'm at the beach

I'm on the phone

I'm in trouble

I'm at my wits end

- "I'm at work" OR "I'm at the office". If you want to be more specific you could say "I'm at my desk" OR "I'm in my office" (referring to your own private room at your workplace).

- "I'm on the phone/computer" and "They're showing the game on TV tonight".

- "I was at home in my room on the computer at 10:00pm".

Prepositions can be tricky. You can find some good examples of how to use them at the following links:

1) http://esl.about.com/library/beginnercourse/bl_beginner_course_gr_prep1.htm

2) http://esl.about.com/od/grammarintermediate/a/prepositions_of_movement.htm ;

3) http://www.talkenglish.com/Grammar/prepositions-on-at-in.aspx

Any question dear Andrea...

Some advanced examples for "in":
- a country in africa
- An island in the pacific
- canards in the newspaper

in & at for places:

in: to a position which is a component of a coherent larger area:

 -I'm in the bed/kitchen/room/balcony/toilet/bathroom/yard -> all of them are coherent & inside the home

at: not to a position inside a coherent area,

 - at home/school/the supermarket/the door ->all of them are components of a town but not coherent, not related to each other