Ah, the million dollar question. I often wonder if that question could truly be answered spending a day training with Sakurai.
Joking aside, still training myself, I don't know how much I can help you. What I will say though is, from my experience, its a lot of going about gaming trial and error style. You don't know what works until you try it.While others may give bulleted lists, I'll try explaining what I think might help.
This is just me speaking though, but if you're a hands-on learner (and gaming tends to be a hands-on learning experience), you can wear your eyes out reading guides and watch videos all day, but if you don't actually take that to the controller so-to-speak, it won't sink in (which then, I recommend gaming with someone who has a higher skill level, and communicate between matches. It may be more effective, getting real time feedback).
Going to tournaments is a personal decision. Its whether you want to train publicly, or train privately. If you feel in any way insecure about your playing, I suggest training privately. That way, you are in the comfort of your home, and you are not vulnerable to any humiliation from the general public being in a public surrounding. Coming from someone who is insecure himself, the first choice can be rather therapeutic.I suggest to anyone asking for advice they train privately from home until they have boosted their confidence in their performance.
As for the character suggestion, only you can choose the character that is best fit for you. I used Yoshi for years, Fox secondary until Smash 4 came out (when I was dissatisfied with Yoshi's performance, but we needn't get into that here).
Keep in mind Smash can be a game of chance and mind-reading, even for pros. The chances of winning aren't as fat as playing the lottery, but they aren't as easy as winning a minigame in MP. Smash is a fast paced game, the gamer constantly having to make split-second reactions (I compare it to two cars coming in for a head on collision at 50 mph each, and trying to avoid that collision). That's generally how fast you need to react on a normal basis, especially online. Otherwise, you can easily be a opponents lunch.
Don't let that scare you though, it'll eventually come as habit kicks in. I'm still aiming for that myself.
Hope this helps.