I recently bought a hugger fan, but I am not sure if the box I have in the ceiling will hold it (Its just the box screwed into some plasterboard). What kind of support do I need to provide for the fan?

Most boxes are fastened to a joist at least at one side. A fan needs more support than that. If you have an attic above the box, you can easily add a brace that runs from one joist to the other, or replace the box with one that comes with a brace. If not, it will be more difficult, likely meaning cutting a large hole in the ceiling and fishing a brace into the hole. Then you would need to patch the drywall. An easier fix might be one of the decorative eschrutons you can buy to cover the hole.

It the box is truly not fastened to anything except the drywall, you may be able to remove it, and add a brace working up through its hole, and replace it. I did a light fixture on a wall recently by cutting a hole for a shallow box, inserting a board through the hole, gluing it to the drywall, and then screwing the box to the board.

or if your attic is accessible you can cut a piece of 2x4 to fit across two ceiling joists and use wood screws to secure to the joists. Drill a 1/4" hole in the center and install a long u or o bolt.. use two nuts with a lock washer between them to keep it nice and tight.

A fan should be mounted so as to be supported by a ceiling josit. It can be screwed directly into a ceiling josit, attached to an appropriate box attached to a ceiling joist, or attached to an appropriate brace attached to a ceiling joist.

For more information on ceiling fan installation:

http://www.ceiling-fans-n-more.com/ceiling-fan-installation.php