Pedaling causes rear wheel/pedals to slip When I start to pedal, say up hill or have to move quickly, my pedals slip and I can't get any movement except for a really bad slip. When I'm on a flat straightaway and have to pedal I hear an awful clanking.
There could be several possible reasons for this behaviour - 


Derailleur could be out of adjustment - can be fixed by barrel adjuster/adjusting the tautness of the shifter cable. You can shift into a cog in the middle of range and visually inspect if derailleur is in line vertically under the cog that chain rests on. When derailleur is properly adjusted, one should be able to spin the pedals backwards freely without the derailleur arm getting stretched/extended.
Your chain could be stretched and/or rear cogs worn. Use chain wear indicator tool to see if chain is stretched and visual inspection should indicate wear on the cogs. Worn out cogs may look sharp and pointy/ like a shark fin.
If you replace the cogs, make sure to replace the chain as well. This helps avoiding undue cog wear due to stretched chain.