Well, first off, CPUs at level 9 read your inputs for perfect shielding and dodging, so they aren't really a fair fight. Don't feel bad about that.
Anyways, for things like terminology, I recommend lurking around (or posting and asking questions), and then when you see something unfamiliar, go to the
SSB Wiki and look up the terminology. That can help a lot, but you have to be aware that not all of it is up to date (or in some cases, even relevant) to Smash4. Checking around on the wiki can also lead to new discoveries. Smashboards is also a good resource.
For characters, there are several good ways to pick, and a lot of it depends on your end goal. For the "average" player, pick your favorite from whatever other source (for instance, Fire Emblem is my favorite series, so I've mained Marth for a long time), and just use your favorite (or best) character from that.
If you intend to compete, typically, much like in pro Dota, you'd go with whoever is the perceived strongest character at the time (Of course, this is a 1v1 or 2v2 fighting game, not a 5v5 strategy game, but the same principles apply). That said, there are some unusual considerations with regards to Smash4 compared to past games.
The meta is new, and as players learn and develop new strategies, a character's viability and matchup dynamics change drastically. On top of that, we've already received one balance-altering patch, and it's not unreasonable to expect more in the future. As a Dota and SC player, I'm sure you can appreciate the effects that has on balance. Also, regardless of what more outspoken community members may say, the balance of aggressive and defensive play in this game is very close to even in viability, so pick whichever style you prefer.
Due to the potential for change in balance, I'd suggest learning whichever character you like the most, whether that's from a playstyle stance or from a source-material stance. Since you're more or less new to the series, you also don't have to worry very much about past character preference impacting your current play. It's always a good idea to play a few rounds with each character to feel if one
clicks with you.
As for improving once you make your pick, practice is key. If your internet is good, then playing For Glory mode online is a good starting point. If not, I recommend playing through a lot of the single-player content and challenges, as those will let you hone your basic techniques up to a certain point. Past that, if competition is your goal, try to find a local tournament scene and start attending. Even if you don't enter, just watching players do fancy things with characters can give you ideas, which you can go practice online or against computers or friends.
Hope some of that helps. Feel free to message me if you want some more advice. I use the same username on Steam, too, and would be glad to chat there, or spar on Smash (my NNID is under my avatar).
