No worries. We all start at the same place when learning the game.
There isn't a set rule that everyone applies but in general you always want to rotate a few guys and not the entire team. I always prioritize by the importance of the match.
In general, the Champions League is the most prestigious European Club competition so you will want to play your strongest lineup unless you have the group stage already wrapped up.
League Play (EPL, La Liga, etc...) is generally the 2nd most important competition so you'll want to play strong lineups but also keeping in mind managing fatigue/injuries/etc...
League Cup (FA Cup, DFB Pokal, TIM Cup, etc...) is usually the 3rd most important competition so you'll want to give some minutes to fringe starters/youth academy kids until you reach the later stages.
League Cup II (Carabao Cup) is generally the least important competition as I think only England has a 2nd Cup in FIFA 21. I always play the youth guys mixed in with a few fringe starters to try to give them some minutes.
In general though, I usually rotate guys based off fatigue and what important fixture(s) are on the calendar. It takes some planning and obviously injuries can happen but by giving your fringe starters some minutes it helps acclimate them to the team and give you a feel for them just in case you need to play them.
I tend to keep smaller rosters with a few youth kids waiting just in case I need cover at certain positions. A lot also depends on what kind of club you choose and what the expectations are. It's a lot easier to break youth into a lower table side than it is the likes of Liverpool, Real Mardrid, etc...
Hopefully this helps. Feel free to ask away. We have a lot of knowledgeable people on here.