The National Electrical Code contains the rules for this sort of thing, plus whatever extra rules your local jurisdiction adds. I bought a copy of the code itself plus an "Illustrated Guide to the National Electrical Code" to learn this stuff. There are lots of YouTube videos for electricians, by electricians. DIY channels can also be helpful, but they don't always follow code.
In my case, the local utility requires the electrical meter to be accessible (obviously) and to be a certain distance from the gas meter (obviously). Because of the way my house is shaped, there simply isn't room to move the electrical meter, so a sub-panel was the simplest option. Anything else would involve tearing open the driveway, which would be worse. It really depends on the situation.
It'll give you a really good introduction to house electrical systems, things like "any panel that is connected directly to the utility is the main panel, any panel that is connected to a panel with a shutoff is a sub panel".
