My client gave me a business card to create an advertisement. I scanned it, now I want to add clipart and edit text. How do I activate the document and make changes? I am using GIMP and Inkscape.
This isn't how graphic design works. You have a raster file, meaning the text cannot be edited very easily. It's essentially a photograph of a business card. See What are the differences between vector graphics and raster graphics? for more information about what this means.
In order to edit anything you either need a working file or to manually make alterations. Let's say the business card is fairly simple and has a black background with some white text. You would need to edit out the white text by painting or otherwise covering it with the black that matches the background.
Likewise to move or scale the logo from a raster file you would either need to mask it (select it and paste onto its own layer) or redraw it in vector. Redrawing it as a vector will give much better results but may be very difficult to do depending on the logo.