An important factor to understand in the prewar world is the Washington and London naval treaties. As the buildup of military equipment will most likely be important in this game players must have knowledge of thier limitations.
The Washington naval treaty was a treaty to limit the numbers of dreadnought battleships being produced, in the end they decided on a ratio of capital ships (US:UK:Japan:Italy:France)
5:5:3:1.75:1.75 these are tonnage ratios, so I don't really expect most players to know the exact tonnage of their ships so instead just work from numbers
By 1938 japan had denounced the treaty, so if someone takes them they can do what they want. Other important things to note are limitations on aricraft carriers as well. In general apply the following rules.
Destroyers and Subs are fine, go nuts.
Light cruisers are ok, but they cannot have guns larger than 6.1 inches, britain has the largest allotment of these
Heavy Cruisers are ok, but their guns must be a max of 8.1 inches, USA has the largest allowance of these
Battlecruisers and battleships are rare, most nations with any will have 1 or 2 old ww1 ones, never exceed six unless you're ignoring the treaty or are USA or UK
Carriers: 1 or two max, no super battleship carrier hybrids (explicitely forbidden)
Ship classes:
Corvette: The smallest sea worthy vessel, the corvette patrols coastal waters, lays and sweeps mines and escorts convoys, usually has one 3-4 inch gun
Frigate: A frigate is a light patrol vessel, designed to escort ships and scout for small fleets. it usually has one or two 4-5 inch guns and little armor
Destroyer: The destroyer is a light general combat vessel, armed with 5 inch guns, depth charges and torpedos its great for escorting capital ships, torpedo attacks and countering other light vessels
Destroyer Escort: trades guns for AA and anti submarine wepaons, escorts convoys and large capital ships
Escort Carrier: Carries a small contingent of air craft for protecting ships from submarines and air craft, generally built on old cargo ship hulls
Light Cruiser: A fleet scout and light general combat vessel, has guns of a 6 inch caliber, torpedos and AA wepons
Heavy Cruiser: Developed in the twenties as a medium combat vessel, most nations built these in lieu of battleships. It lacks the power of a real line of battleship but instead has speed and a dual combat/scouting role. usually found on secondary fronts.
Auxiliary Cruiser: A repurposed cargo ship with guns, good raider.
Panzerschiffe: A german design, the panzerschiffe combines the guns of a battleship with the hull and speed of a heavy cruiser. Counts a a battleship for purposes of the treaty.
Pre-dreadnought: An old design, the predreadnoughts are battleships constructed before 1906. They have smaller guns and less armor and a varrying miz of gun sizes, making them unwieldy and only really good for shore bombardment.
Battleship: A true dreadnought battleship is a nation's primary combat ship. They are few, so treated like a strategic resource and only used in important battles. They are well armored and powerfully armed, but generally slower. Modern desings are rapidly getting faster.
Battlecruiser: Another older design, the battlecruiser is battleships size and battleship armed but lacks the armor. its primary purpose is to run down and kill cruisers, and are generally faster than anything bigger than a light cruiser. Unable to go toe to toe wuith a real battleship, they excell at raiding, hunting smaller ships and fighiting in adverse weather conditions. Their speed allows them to close with carriers in poor weather and kill them. They are, however, fragile.
Light Carrier: Light carriers are a japanese design and mostly just small aricraft carriers.
Aricraft Carrier: A floating airfield, simple as that.
Ship numbers as of 1938 (we'll assume you have sufficient frigates, corvettes and destoryers)
Spoiler for Japan:
4 battle cruisers (Kongo, Hiei, Haruna, Kirishima) 6 battleships (Fuso, Yamashiro, Ise, Hyuga, Nagato and Mutsu) with 3 super battleships in construction (Yamato, Musashi, Shinano) 7 aircraft carriers (Hosho, kaga, Akagi, Ryujo, Shinyo, Soryu and Hiryu) many more are planned and in various stages of completion. You have 17 light cruisers and 16 heavy cruisers. 2 more heavy cruisers are almost completed, as well as many light cruisers.
Spoiler for United Kingdom:
The royal navy has 12 battleships (Revenge, Resolution, Royal Soverign, Royal Oak, Ramilies, Warspite, Barham, Malaya, Queen Elizabeth, Valiant, Nelson and Rodney) 3 battlecruisers (Renown, Repulse and Hood) 7 aircraft carriers (Eagle, Hermes, Argus, Ark Royal, Corageous, Glorious, Furious) 30 light cruisers, 18 heavy cruisers. Many mroe lgiht cruisers are planned.
Spoiler for United States:
The US has 15 battleships (Arkansas, texas, new York, Nevada, Oaklohoma, Pennsylvania, Arizona, New Mexico, Mississippi, Idaho, Tennesse, California, Colorado, Maryland, West Virginia) Four more are planned to being construction in 1939. They also have 6 aircraft carriers (Langely, Lexignton, Yorktown, Saratoga, Ranger, Enterprise), 17 heavy cruisers and 12 light cruisers, with many more planned.