most secondary peaches are looking for her use for teams, brinstar, or ic's. Idk if you wanna use her for these reasons, but here's the skinny of it:
Start by understanding float canceling. It works like so: if you start an aerial while in your float and then lag, there's no lag. The things you will probably do after dropping fair are the following: jab, fullhop/shorthop nair, dash attack, dsmash, grab, and wavedash back. Figure out when to use each option, and how to mix them up based on what you're doing post fair. Most of the time, however, you're going to want to drop fair into dsmash.
Avoid forming habits post-fair. Many peaches will jab, dash attack, or dsmash after every fair. You need to follow fair not with what you want, but with what they don't want.
Learn how people move around your turnips, and how you can use it to combo into fair. Know what you should do once you fair their shield. This and the above are crucial,
peaches metagame is based around covering options and getting small punishes that result more in positioning advantages (for closing more options) than combos or large percent punishes. It's like knowing whether or not to chaingrab them or throw them off for a ledgeguard. You'll be in a much better situation if you ledgeguard.
learn basic ledgeguards for the spacie matchups and marth. Peach needs ledgeguards for kills in those matchups, so it's crucial to get them down. For fox and falco you're gonna wanna float and bair, but get used to the idea of dropping your float and reacting to them doing an option you didn't expect.
Don't spam dsmash :3 especially not on platforms. More over, **** them with it if they're getting hit my it, but you can't make it such an integral part of your game that you begin to lean on it when you panic. Peach panics easy.
Agression loses matches as peach. Your approaching game is already weak, you want to close out their space and dash attack confidently when you do approach btw, but
the best advice a simple peach will hear is "float and wait for them to **** up". Most people just get impatient as you float and will throw out moves that will never hit. Punish with fair :3
Dsmash is a good move! When you use it, make sure you're doing it consciously. That means, think about if it's the right move for the situation, if it'll hit them, if they're predicting it, if they're able to avoid it (frequently they won't). Dsmashing on autopilot does you very few favourites.
C'haingrabs aren't too important seeing as the situations i listed above (teams bstar ic's) won't provide situations to chaingrab. In teams, your goal is to do one of two things: Play backup or agress. Peach does both pretty well. When backing up, you want to mostly cover your teammate, which peach can do pretty well by floating safely and then dropping with a high priority, high knockback aerial that will keep them away. Remember in teams, the only lag you have is dash attack and dsmash, so you want to use those moves when there's a bit more guarantee about landing it. Fair and bair can be used for a very good turtiling game in teams, staying safe and keeping them away while you 2v1. I generally backup the spacies, ganon, and marth.
On the other hand, I prefer to agress in teams with players or characters who require one setup for a kill. A good example of peach agressing in teams is macD and Zhu, macD jumps in with high priority, safe moves, his primary goal being to be difficult to avoid and punish. Zhu pulls them off or disracts them enough to give macD a free hit, which allows zhu to combo him to the ledge or death. Agressive peach in teams is that: be hard to avoid and punish. Use fast float cancels, if you see anyone run in, cover yourself with nair jab or dsmash. you want to force them to tech here. If they roll away from the carnage, you want your teammate to be there to grab and then do their stuff while you **** the other guy. I like this with jiggs (dsmash to techchase rest, also peaches laglessness makes fighting off 2 people easier than jiggses massive horisontal aerial movement.), Sheik, and C. Falcon (peach and falcon is just a godlike team. Peach can take a spacie at zero and instantly set him up a tech chase at 40, peach can help him cover multiple options while he has to tech to avoid the dsmash and the knee. Tons of bad di)
If you dsmash on brinstar or vs ic's, you will win. Well, if they either don't know what they're doing vs you, or screw up at all, then yeah, you will win.
This is also good info, KK too good:
Fair is a heavy, slow, high-priority, medium-range move that controls an area in front of you (and slightly above / below you). Its range to priority ratio makes it very threatening as a move, and deters approaches. This is a move you use to control space. It also beats a lot of stuff, so if people come in on you when you're spaced well (defensively speaking) it can **** approaches. It also applies a lot of pressure to shields. This is what you do when your float spacing is setup (vs a lot of characters; not Falcon though).
Nair is good when they're above you, or if you're in their face. It's also good if you need speed. It's her fastest aerial. It has some range in front of her, but not that much. It has some hitbox above her and behind her (where her arm swings). Can be good to retreat with against characters with medium / low range (FJ Nair backwards has good defense properties) and you can use variations of it (mostly float, but also SH) to sneak underneath characters like Jiggs and Sheik who depend on a specific positioning.
Bair is good for coming from above because of where the hitbox is positioned. It's sort of a middle ground between Nair and Fair in that it has more priority than Nair, but is significantly faster than Fair. You can do similar "sneak under their hitbox" tricks vs characters like Sheik that you do with Nair, but it has the benefit of more range (which is nice). Can be used to start shield pressure, but not as good as Nair to continue it (slower startup). Good to zone vs low range, defensive opponents because it's awkwardly hard to challenge.
Bair is the best for edgeguarding and gimping because it's the easiest to setup. Fair is good if you can hit it (mostly at low percents) and Nair is clunky and trades too much with stupid stuff (but ***** if you can time it through their hitbox, or if they don't have a hitbox).
Feel free to ask if you've got any questions. :3 One last thing: Bring your personal style into it. One of the things I love about sub peaches is they all serve different puropses, and their styles reflect who they main. The way peach is made leaves many things open to interpretation, so don't be afraid to think outside the box.