The basics of sewing are really easy. Just start doing it and it'll come to you. It'll take some time to get things looking nice, but if you're like me you care more about functionality anyway
It's completely an accident that my stuff has started getting more aesthetically pleasing!
Honestly a hammock is about the easiest project to sew. I can make one in about 20 minutes at this point. Just four hems. If you're nervous about having your first project be load-bearing, you don't have to use it right away!
Jeff's all-in-one is slightly more complicated, but not much. It took me way less than an hour to make mine. And you don't have to rely on it to support your weight!
Snakeskins and stuff sacks are a little more complicated yet, but also good intro projects.
For info on how to make any of this stuff, check out Jeff's homemade gear section:
A rectangular tarp with straight edges and basic tie-outs would be pretty easy. And less-than-ideally functional IMO, but that's just me. I like the hex-cat design (obviously) but it's more complicated in several ways, as well as requiring a higher level of sewing proficiency.