I have to build flowcharts. Where can i find the symbols? Does there exist a library, which i can download and use, or i have to draw all the symbols for myself?

For arrows simply draw a path and select "Fill & stroke -> Stroke style -> End markers", if you need something more advanced - try Dia, it has big built-in set of objects for flowcharts.

The Flowcharts i am used to only incorporate ovals, triangles, squares, squares-standing-on-edge and circles. And of course Arrows.
The Ovals are best done with - simply drag the little circle at the edge to the middle to make the edges so round it looks like an oval.
Triangle, Squares and Squares-on-edge are best done with select the number of corners, then press Strg while making them, and you can rotate them in only a few angles.
Circles with , of course, and gives you the connectors. (Note the controls for the connector tool at the top of the canvas)
Also, note that you can define in preferences whether you want to draw one object in the style of the last, and you can copy styles (Border, Color, Filter and all) by copying the Object, then Strg+Shift+V onto the "victim"

Try openclipart.org for symbols...

I've seen some really nice flow charts created with Inkscape, but the last time I created a flow chart I wanted to create it as quickly and easily as possible. So I used OpenOffice.org Draw.