Just going to through this out there: If anyone needs a WiFi practice buddy I'd be more than happy to oblige.
As for essentials in Pikachu... hmm... Well here is what I have found most helpful and why.
QAC (Quick attack cancel)- Because not only is it a great recovery option and it keeps you "unpredictable" but it teaches you to control the quick attack. That's what's most important. Recovering from being thrown off the ledge is probably the most difficult thing any newcomer faces, but with Pikachu it isn't too difficult. He has Skull bash and Quick attack for recovery options. Just make sure you are aware you can Skull bash as many times as possible in the air, but once you QA you go into a free fall and can't preform anymore recovery moves. My recovery usually goes Jump>Skullbash>QA. I could go on for an enormous amount of time about the applications of QA, but I'll end it here (if you want to know anything more about anything Pika, just message me and ask)
Do NOT force the kill- I usually end up killing CPU's and Friends after 150% damage. Usually because I am not TRYING to kill them. Pikachu can rack up a lot of damage, fast. And the best part about him is he's quick. You don't need to try to kill anyone. Your goal shouldn't be to get KO's, but rather give more damage than you take. A lot more. I have given 514,000 Damage and have only taken 216,000 (yes I just looked). Worry about damage, the KO's will happen, but usually in the higher percentages. Which is fine as long as you can prevent yourself from accumulating damage.
Don't Stale Moves- The game is kind of funny. If you keep doing the same move, it becomes "stale" and a stale move isn't nearly as good as a fresh one. A fresh thunder at 100% can KO, while a stale one usually will fail to do so. A barrage of headbutts hitting an opponent usually un-stales most moves. First of all, if you are playing against a friend instead of the AI, if you don't stale your "power moves" like thunder, Iron tail, N-air, and so on, a surprise Iron tail when they get to 120% would be far more likely to hit. Which brings me to my final advice...
Be unpredictable- This is the most crucial advice I can give. It may seem difficult at first, but once you learn that Pikachu has a vast amount of options both in air and on the ground to keep the pressure up, you will have a far greater chance of winning. Air attacks while you are on the ground are a prime example. Nobody expects a F-air when they are standing on the ground, but learn to preform it. You'll not only evade most attempts at a preventive approach, but you'll keep that unpredictability up. This is the most important thing I've ever learned, and it seems simple but in practice it is difficult. Eventually you'll get the hang of it.
The only other thing I would say is practice. Some people do things differently. I, for one, use T-jolt (neutral special) all the time. I use it as I approach, recover, flee, camp, everything. It keeps the pressure up and I like that. But some people don't enjoy playing like that. Find your play style. Best of luck, and if you need anything further please do not hesitate to ask.