While you’ll struggle to compete with Britain, having a decent surface navy (a mix of destroyers, light cruisers and battleships) will help you in naval invasions. Moreover, if you go democratic, non-aligned or communist you will need a strong army to defend against Germany – you should focus on infantry, forts and naval production as it will be easier to compete with the Germans over Britain’s immense navy. If you go fascist, you will need at least 24 divisions (18 infantry and 6 motorised) to get a respectable war score in France. Following these will give you a greater chance of success.
As usual, I will begin the guide with some rudimentary principles that are worth following, even if you ignore my military tactics and political ideology.