In reply to saturn:
Firstly, if you're new to climbing, roped solo probably isn't the best way to start. It's an option and I'm sure other people have done it, but you'd really be better off looking to meet up with some other climbers who can help you get started. Climbing Facebook groups for your local area could be useful, or the lifts and partners section on here. Not everyone will want to go out with a beginner but there are a lot of helpful people in climbing who enjoy sharing skills.
I realise that starting an answer by telling you that you're asking the wrong question is a bit of a cliché but I think it's warranted. Your questions give the impression that you're lacking quite a lot of foundational knowledge that would be necessary to rope solo safely. You might get some useful info here, and there are other resources online that go into detail on setups and options, but without some basic knowledge and experience of your own on anchor building, you're not going to realise if you're making mistakes that could have serious consequences.
> 1) Is anchoring to a tree with a figure 8 knot OK? If not, then why not?
Depends on the tree! But also, you'll be lucky to find very many climbs that have a good solid tree directly above them, so you'll need to know how to be flexible and use other options. On top of which, a single-point anchor is never going to be particularly stable, which increases the amount of back and forth movement of your rope on the edge, which isn't good.
> 3) I was thinking of getting a microtraxion, will this alone be adequete to ensure my safety if I fall? Have they ever been known to fail?
Any device can fail in some circumstances. The textbook way to top-rope solo would be to have two different devices on two independent ropes. Plenty of people don't bother doubling up, but it is a compromise on safety.