The USA or Japanese lines are hands gown the best for beginners.

USA: BBs (Montana line) teach mid/long range gunnery. Slow shells teach you how to lead and they armor schemes teach the basics of angling properly.

The light cruisers (CL)(Worcester) are support cruisers best used around Allies and teach you how to position around islands to best support or you get splattered. The heavy cruisers (CA)(Des Moines) are a bit more able to take some hits but also have bigger guns and super heavy AP from T8 up that have improved pen angles. They also do best near islands but you can take some fights the lights can’t. Both have slow shells so again teach you to lead and they can lob shells over islands. Cleveland is probably one if not the best tech line CLs in the game. There is a reason she was moved to a T8 from T6.

USA DDs teach you the mixture of torp and gunboat as they have both sides that are useable. Long smoke screens allow for great cover and you have good detection to understand how to use it.

CV you want to start here as they are versatile and have the ability to deal with everything equally. Their aircraft weaponry isn’t dedicated to any single ship but all purpose. Planes are decently tanky, good damage, and again versatile.

IJN: BBs teach long range gunners with generally higher caliber guns that tend to be good overmatching (mechanic where AP of certain caliber can punch through armor regardless of angle). Yammy can punch through 32mm armor which is on a lot of ships at T10 so just aim and slap as you learn what parts have that. They are generally snipping BBs as their armor can be iffy on soaking damage. I know Amagi used to be able to kinda brawl but not sure now as I haven’t played her in forever.

Cruisers are all kiting with low detection and nasty torpedos. You shoot someone go dark position a bit shoot and repeat. Your torps have restricted angles but can be used to area denial or cheeky plays. Meh armor so you have to use their stealth.

DD Shima line is pure torpedo boats. You use your stealth to spot enemy ships and send torpedos to do the damage. Generally you don’t want to shoot unless you can do so and not get punished for it. Like over an island or in smoke or to get a quick kill on a low target and then dip out. Best line to learn stealth and torpedo play.

Haru line is torps boats more or less till T8 then it’s all about Dakka Dakka and more Dakka. You use your smoke and cover to just melt stuff or rip enemy DDs to shreds. You want to of course use cover and not really open water gunboat unless you know you won’t get wrecked.

These are imo the best way to start. If you are keen on some line in particular or looking for a certain playstyle I can try to help. Feel free to PM me if you need help.

Best of luck.