I have some free vector icons. I need them with a transparant background. How can I do that?

Depends on how it was made.
If it is a vector image, just remove the white background object.
If it is a raster image, you either redraw it entirelly or try to mask or clip the background,
and maybe it's easier to do that in a raster editor program, like gimp.

Thanks Lazur, it's a vector image. Can you explain how I remove the white background object? I can't find it in the handbook.

I could better tell if I see the svg.
This is now driving blindfolded.

Those object can be on a locked layer,
the background could be on a locked layer alone,
the object could be grouped together,
the object can be turned to a fill pattern with a white background and added to a rectangle,
the white parts can all be made from the background white object.

You can check the layers panel (Shift+Ctrl+L),
look at the indicator bar when you select objects -with the node editor tool,
switch to outlines only display mode to see how it was made.
Maybe a look at the xml editor can help telling how the objects are put together.

Attach the svg if you want some more specific solution.

If you're trying to create a PNG file, make sure you use File > Export Bitmap, NOT File > Save As...