UK = easy mode and friendly for Beginners

MN = short range but fastest of all nation

RM = average range, low hangtime shells and still OP nation :D

SN = 2nd best range, 2nd high hangtime shells

IJN = Best in hunting ships with their Subs, best in ruining a Blitz with a Kita kami, many choice of cv to play (they got paper planes though), nice BBs (can take out an AW ship without AP as long you fire it at max angle)

RN (UK): Good ships, good damage dealer, average range, receive a hell of a beating, expensive armor. Good CV (Carriers), great Torpedo bombers anf fighters. Dont know about Subs.


KM (Germany): Best range in the game, average to good damage dealer, average armor, weak ships. Great CVs, good pilots in general. Deadly Submarines.

RM (France): Good ships, good to great damage dealer, recieve a hell of a beating, average armor, bad range (not the worst, but bad). Average CVs and average pilots. Good Submarines

SN (Russia): balanced all around, good damage dealer. Dont know about submarines.

MN (Italy): Low angles for massive damage, great punch, same durability as RN and RM.
Good CVs, good dive bombers. Dont know about Submarines.

IJN (Japan): Good range, good damage dealer, good durability, deadly Submarines, good CVs, good pilots in general. 

USN (USA): Balanced all around, great CVs, great Fighters, good submarines.

Thats what ive seen, played and tested in test server last year. 

Ok so it's like that.

BB5

USN - Can oneshot everyone but has low range.
RN - Smaller than most and can AW. 
IJN - Can pierce RN armor at max angle. Fast.
KM - Haha... sigh.
RM - Low angle rushing ships.
MN - Does what montana did but even better.
SN - nearly KM range, good spread, slow speed

BB6

USN - Best BB6 in the game.
RN - Low angle, tight spread, can AW.
IJN - Very high damage, high angle, long hangtime, can (usually) pierce RN armor ridiculously lethal 6.1" AA battery.
KM - same as  KM BB5
RM - Same as RN but without armor
MN - Same as KM BB5
SN - God of HA defense.

here are the subs in terms of sub fighting ability, ss5's

US: slow turning, no armor space, fast reloading torps, alot of torps, high dud

UK: few torp tubes, average turning, average armor, fast reload, very few torps, high dud

KM:  tight turning, proxys, lots of armor, fast reload, lets of torps, low dud rate

IJN: slow, slow turning, high dud, lots of torps, 6 tubes, average armor

MN: too long, slow turning, high torps, average duds, 

SN: tight turning, fast torps, average armor, lots of tubes, slow reload

RM: lots of tubes, highest turning, very thin, average reload, lots of armor, high support slots

since you said that you are new to the game there are some things you should take note of.
CVs can generate a good income in credits if you have good ftrs (us and the km are the best atm). Also CVs cost a lot of credit to buy the ship, so it would be wise to start a bb crew first, or even raise an extra pilot with the one you will be raising for yourself and sell him so you can have enough money to buy the cv you want to.
BBs you always need to think about your last bb6 when planing your crew. You will need a total of 14 sailors.So a captain a scout a pair of main guns and the rest is upto you. Check each specific forum for crew tips. You are better off raising 2 captains for bb line as support sailors tend to drop behind the lvl of your captain. This way you can have your crew at the same lvl as your bo.
SS i would suggest to first try the 2 above lines before going for an ss. If you decide to go for an ss then you are better off choising the same nation as the rest of your sailors so you can use the supports you already have.

Nations:
KM (german) hard to play with bb if you are new. Good range but low damage output and massive hitboxes. SS leathel for all other ss (ss-hunters). CVs very good ftrs nice dive bombers but they all luck fuels.
US(USA :)) good all around. Ballnce nation. BB wise you get the best bb6 in game, you have a leathal bb5 (short ranged though) and lots of ships to play around. CVs they have the ebst ftrs and their torpedo bombers can be considered stealthy due to the color of the planes(same as the ocean color). SS pretty good all around (lots of torpedoes).
RN(UK) BB very good damage ouotput(second best i think) easy to play (noob friendly) can take lots of beating especialy if armor whore (armor cost losts of credits though). CVs not so good ftrs (durable though) best torpedo bombers (damage wise). SS no idea.
SN(Russia) BBs second best range although the shells take some time to land  so you need to predict a bit as to where the guy will be when the shells land. Very good damage output compared to KM. CVS have the best dive bombers in the game. SS and FTR no idea :).
MN (France) BBs very fast and agile ones, best damage output but very low range. Small hitboxes of bbs so you can rush in and out. CVs and SS no idea
RM(Italy) smallest bbs in game. Very fast and agile just like France but more leathal due to the low degree guns. Considered OP although i doubt a nerf will come into effect. CVs and SS no idea.

The above are personal experiences from the ship lines i played. If someone feel like i am mistaken on something please go ahead and correct me.

Personal opinion i would suggest to you to start a UK BB to get to know the game. Once you feel confortable enough with your skills go aheadf and try some other nations. Also keep in mind that the game play till lvl 60 (blitzkrieg) is completly different from the rest of the battle modes. Great Battle is the mode of choise for most players due to the shared experience. But try to stay and enjoy the first lvls (upto 60) in blitz as anywhere else you will be food for the bigger ships.