Welcome Detzi!
I was going to suggest using Path menu > Trace Bitmap. I thought it would be possible to adjust the Threshold for the Edge Detection option, and get a good result. But I can't get anything that even approaches reasonable.
You might be able to adjust some options for the scanner, to get a better result. But you'd want to be sure and revert the settings when you're done, or else it will affect everything you scan.
In this case, I would suggest manually tracing it, using the Pen/Bezier tool. I wrote a tutorial which explains the basics. (It's meant to help people who are planning to cut out their drawing with a digital cutter, but you can skip the part about making an outer border path....well, unless you're planning to cut it out.)
https://forum.inkscapecommunity.com/index.php?action=articles;sa=view;article=45If you're planning to do a lot of this - converting your hand sketches to vector, you might want to consider getting a graphics tablet. With such a device, you can draw the sketch directly into Inkscape (using a graphics pen, rather than mouse)! (Graphics pen and tablet is not the same thing as and ipad kind of tablet. Although it seem the line between is getting blurry, and maybe there are some hybrids that I've never heard of. I don't exactly keep up with the market for these things.)
