I'm not playing online until I know the differences in all my attacks (and which big ass robot I want to use).
Temjin is the usual recommendation for beginners. He doesn't specialize in anything, but by being generic he's probably the easiest to learn. However, most of the VRs are fairly unique, so try them all out... one of them might naturally click with you even though they're technically harder to learn.
For a beginner, the only ones to avoid are Bal Bados (by far the most complex VR), 10/80 Special (intentionally underpowered), and Ajim (overpowered boss character, but underpowered as a playable character).
Also, here's a tier list
in case you're interested. I settled on Specineff before I saw the list. I was surprised to see him rated at the top since you don't see too many Specineff players in the lobby.
Lots of attack variations are useless most of the time though, eg. crouch dash attack variations are rarely used and lots of the melee attacks are impractical.
I actually use sliding attacks fairly often! But yes, most attack variations are largely unused. I think the best thing to do is:
1.) Try them all in training mode first against a stationary opponent. This will let you know what they all do.
2.) Them try them in Arcade Mode or Score Attack, this will help whittle it down to the most useful attacks.
3.) Play against other people. If possible, choose people that aren't too good at first. Even when playing against chumps you'll learn more than you can while playing against the AI.
Also, these charts
might help if you're trying to figure out how effective all of the variants are. It's based on 5.45, not 5.66, so it might be a bit off, but it should be close enough. Unfortunately Stein-Vok, Apharmd C, and 10/80 aren't there since they were new in 5.66.
Still cant get the hang of close combat though.
Unless you're using Apharmd B, I think your time would be better spent learning mid and long-range combat at first, since that's usually where the meat of the game is. Although I might be biased, since I royally suck at CC. I generally just try to stay out of CC range.
EDIT, forgot this:
I had a few matches online for the first time the other day. only won 2 out of 11 matches but I learnt a lot about dodging fire.
My initial record was pretty bad. I think I was like 13-42 or something like that. And that was around launch, so the overall mix of players was more noobish at the time. I played the DC version a lot, but I was rusty, and I didn't have any experience against human players.