How to come up with a good webcomic plot?

I'm 16, and created this webcomic a few weeks back. It's about two best friends and their experiences in the world. But it doesn't seem to have much of a hook when it comes to plot. I cannot think of what to write my strips about most of the time. Others seem supportive of my idea, yet I cannot come up with enough stories. You can see what I have so far below. How can I establish a decent plot?

http://josh-angus-adventures.thecomicseries.com/

By the way, I keep getting D/E/F grades in GCSE English Literature at school. Does this have something to do with why I can't come up with anything?