Once you have posted an assignment, you will be able to see how many people have submitted work to you and how many people have yet to upload anything.
If you click on your assignment you will be brought to a page that shows you all of your student's individual submissions, sorted by who has submitted work and who hasn't yet. You can click on any student's name to see their submission.
After reviewing a student's work, you can then choose to add a mark out of 100 in the 'Add grade' space beside their name. When you have finished marking, click the tick box beside each student's name and then click 'Return' on the top left. This will send your grade back to your student.
One final thing to note is the small 'Hamburger' button on the top left of your screen that looks like three dashes. Clicking on this will bring up a sidebar that allows you to go to your 'Work' page that will show you all of the assignments that you have set across all of your classes.