Fundamentally creating a game with a one man team isn't really any different than creating a game with a huge team other than the fact that you don't have as many (or any, really) specialized people or parallelization of tasks. The decisions you would make here are the same decisions you would make if you were running a team who's goal was to get to ship as quickly as possible.
So you have to play to your strengths and design away from your faults. If you aren't very good at high poly art, you either need to pick a style that works with your skill set, or use existing art in the form of mods or buying art off stock websites. If you don't know anything about multiplayer networking, use middleware that solves most of the problems for you.
In particular, getting bogged down by non-critical-to-gameplay technology and features is going to suck away your time and not get you closer to your end goal. I'm personally a big fan of Unity due to it's super quick iteration time, easy art pipeline (just drop in pretty much any format and it'll work), and the ability play the game from within the editor. All of these will help you get up and running quicker than using something more "professional".
What programming language should I use? What modeling/animation software should I use?
Use whatever you're comfortable with. If you're not comfortable with any of them, just pick one of the popular ones. The choice of tool here isn't going to make or break your progress by any significant factor compared to the design decisions you decide to go with.