New Here & Trying to Build a Campaign (bad combo?)

I signed up with Roll20 because I thought I could use it to help me build a campaign. I have a text-only document skeleton of the entire plot, some mad ideas, and a $0 budget... I went through the tutorial just fine (I think) but once I got to "Create a Game"... I can't access the editor any more. Is it a tablet-only thing? I want to make maps and write flavour text and all that good, good module/book horseshit[Including NPC's, tables, and assorted whatnot]. Is there a way or have I picked the wrong toolbox? Am I being too ambitious for someone who's only played like 15 sessions of D&D? [I do expect this to take a while because learning curve, so I expect this to maybe be nearing readiness by the time our group's current campaign is done with] TLDR: Halp I am smol and lost and confuzzled.

"Table on Tablet" is a subscription feature. To use the site at the free level requires a laptop or desktop computer. And to be honest, the performance and behavior is much better on a computer as well. To answer your other questions, yes, all those things are certainly doable, even at the free level. I started my campaign that way.

I must be an uber-dummins because I can NOT figure out how to access the map editor for free here. Got a step-by-step on Grandma Level guide for getting into it? I already have a game created, but I haven't launched it or invited anyone (because there's nothing there). How do I make a map in the editor from there?

You have to launch the game.  Creating characters, setting up maps, creating handouts, etc. are all things you can only do in the game.

Maps are made by either uploading graphics  (which counts against your upload limit), searching for free ones on the web via the (very limited these days) images tab or using purchased items from the marketplace  (these won't count against your upload limit which is visible on your account page ).