I have a question about an expiring domain.

First off please bare with me as i never dealt with expiring domains before.

So, i found an expiring domain that's has been ranking pretty well until January, and is still ranking for many long-tails as of today, even though it has been parked by godaddy more than a month ago.
The domain doesn't really have good metrics. PR0, DA20, Both TF and CF are 0, has lost a lot of links in January. The few remaining links aren't that strong, but somehow it's still ranking first/second page for plenty of keywords.

My question is: How do i register this domain? I understand that Godaddy has a 45 day grace period where the owner can still re-register it, then it goes directly to public auction? What happens if i backorder it?

The way you proceed in this matter is by backordering a domain. Chances are if you wait until it becomes available to the public, someone else might grab it. Hence the reason you use a quality backordering service. There are many backordering services and first off, don't use the one GoDaddy offers unless the metrics are crap (in this case it may be ok)

Try using DynaDot.com. Their service is pretty good at catching decent domains. If the domain has PR and crazy metrics, use a more popular backordering service such as snapnames.

Also, based on the domain metrics you provided, I would like to warn/caution you from buying this domain. It's not because of the metrics but because the backlinks are disappearing from the domain. This could be due to the fact that they were software-generated (web 2.0s, guest posts,etc) which is a bad thing.

Run the existing backlinks and make sure they are niche-related.

Hope this makes sense.