You'll need to be a little more specific, what kind of beam is it, what effect do you want when it's covered, will it still be visable once the ceiling is done, etc. Post back up with more details and we'll try to help out.

I assume it's an I-Beam. You can build a 2 x 4 box-in that covers it. Your drywall/plaster will attach to the box-in. The box-in will attach to the ceiling joists.

Side view of I-Beam:
The A's are 2x4s
The B's are the drywall/plaster.
The x is plywall that connects the top 2x4s to the bottoms. The rest is the I-beam.

BA |-------| AB
B x | x B
B x | x B
BA |-------| AB
BBBBBBBBBBBBBBB

In the last reply, the spacing got messed up in the post.