Hi!
You get S only when you full combo(FC) a map. you get A or lower grades when you miss.
The information below is complex for beginners so don't feel bad if you can't understand all of it.
Now let's talk about PP. PP is the thing that determines your global rank.
Maintaining high accuracy is important if you want high PP. Generally, try getting 98%~99% accuracy or higher.
Each miss make you lose PP. Sometimes high accuracy can make up for that loss but try to FC whenever you can.
Instead of passing maps that are too hard for you with low scores, go for high accuracy and FC instead.
Hitting big notes doesn't give extra PP.
Mods give extra PP but I recommend not using those until later on. Instead of playing with mods, try to improve basic skills first.
Now that's talk about scores. A play's score determines your spot on that map's leaderboards. It's also important in tournaments.
The scores you get per note (300) start from 300/note, increasing every 10 combo until 100 combo (1100/note). Because of this, misses that are close to each other (<100 combo gap) or at the start/end of a map loses less score than those that are separate.
Hitting big notes with both hands simultaneously gives extra score if you time it right.
Mods also affect your scores. Again, I recommend not using those until later on.
Last but not least, tournaments. There are quite a few tournaments designed for beginners. Don't be shy.