I'm a little late to this thread, but if you're still in need...
Assuming you already have basic stuff like short hops, shffls, l cancels, jc grabs, etc. down (and by down, of course i mean 100% consistency, because you should not move on to harder stuff until you have this) there are a couple things I highly recomend you learn to build up your punish game
-Waveshine>upsmash/grab. This works on such a huge portion of the cast (sheik, marth, falcon, peach, mario, doc, yoshi, dk, bowser, ganon, roy, link, ic's, zelda, and luigi if you are really good) and has so many set ups. Drill shine and nair shine become infinitely better tools when you can true combo them into upsmash. Especially if you are playing against lower level players who don't know how to sdi your down air, this technique is going to take you sooooooo far. Here's a video example
https://www.youtube.com/watch?v=K8n2MNfQDYk
-Thunder's combo (shine>perfect wavedash>jab reset). This combo works on all characters that are knocked down by shine (basically all of the characters not listed for waveshine upsmash). Mostly this will be helpful against fox and falco. Note that this combo only works if your opponent does not tech the shine, because you can only jab reset someone on a missed tech. The most common way of applying this combo is dair>shine>perfect wavedash>jab reset>shffl upair>upsmash. Yes this is a true combo. The only ways to escape are to sdi out of dair, away from shine, up from jab, or up from upair (although these are really hard) or like i said earlier tech the shine. Here's a video example
https://www.youtube.com/watch?v=XSmoWo5nlYI note that when people say thunder's combo, they really just mean the shine>jab, as the rest is kind of extra.
As for your neutral game, my two biggest tips are to abuse the following
-Circle jump (short hop forward>waveland back). This is really good if you've conditioned your opponent into thinking you are going to short hop nair/dair to approach. It will bait them to use a defensive option (normally this will be an anti-air like uptilt, wavedash back, or simply a shield if they have good oos options like samus) most of which you can then punish with a simple jc grab. Also, it looks sexyyyy
-Wavedash down. See Lucky's guide on ssbm tutorials
https://www.youtube.com/watch?v=ojnkaPiyfRg
