Welcome to Solidworks. You will soon find there are many ways to achieve this. With just 5 hours experience you're probably better using the basic 2d features for now.
When you do an extruded cut the default extrude direction is normal to the sketch plane, but in a 3d sketch or optionally in a 2d sketch you can specify a direction
Please see this example, good luck...enjoy!
SlantCut.SLDPRT.zip 84.1 KB