How can I create a trapezoid in inkscape?

> Are their other preexisting shapes that I can use (i.e. trapezoid) in inkscape?

No - currently Inkscape does not offer shape libraries (it is a known feature request, but not yet implemented).

You can modify a rectangle easily to create a trapezoid:
1) draw a rectangle (with sharp corners)
2) convert the shape to path ('Shift+Ctrl+C' or 'Path > Object to Path')
3) switch to the node tool ('n' or <F2>)
4) select the upper two nodes (draw a selection frame, or 'Shift+click' on the second node, or click on the path segment between the nodes)
5a) use the keyboard shortcuts ',' or '.' to decrease or increase the distance between the two selected nodes to create a trapezoid [1].
5b) drag the selection of nodes (grab it at one of the highlighted nodes) to create a parallelogram

Pressing 'Ctrl' while dragging nodes with the mouse will constrain it to horizontal or vertical directions. Additionally, snapping (e.g. nodes to grid) can help when precise dimensions are needed.

Other basic shapes can be created with the star/polygon tool: choose the number of corners and draw in 'Polygon' mode to create variations of regular polygons. You can always convert such a shape (polygon, star, rectangle) to path and use the node tool to adjust the position of the corners/edges as needed.

[1] <http://inkscape.org/doc/keys048.html#d0e3637>