You need to use Categories (which has been a bit of a headspin for me, too - Completion is a piece of cake compared to this!)
First have a look here http://docs.moodle.org/26/en/Grade_categories
I'm just going to say that when you get to the bit 'Editing the Grade Categories' there is a massive pain, which is that it's just about impossible to see across all the fields - you have to scroll back and to, and (unlike a spreadsheet) that means you can't see both the first item, and the tickbox at the end. Use 'simple view' when you come to adding categories into the gradebook, and zoom out. Don't be fooled - like me - into continually ticking the box in the middle column of the table instead of the one at the end.
I created three categories 'Level 3 graded task', 'Level 4 graded task' and 'completion required' (the last one is to do with ensuring the people on the course post in forums) Once you've created the categories, you need to move the tasks into the appropriate category - which is when it gets difficult (see above) so sort out some categories immediately, and assign each task as you create it!