As a united Japan you have a lot of cored provinces which at the very least will give you a very strong economy with no revolt risk on the island. At this point you are stronger than England and France combined but you will fall behind technologically quite soon. If you want to be competitive in technology, start getting your country ready for westernisation by getting your sliders towards Centralisation and Innovation. During this time, conquering mainland Asia and colonising the Americas should both be an option. If you just maintain a decent home army and a strong navy you really can't go wrong so feel free to mess around and get a feel for all parts of the game.
Asking questions about specific game mechanic here will help too. For example, to get troops onto boats you will need to build transport ships (cogs, in the early years). 1 cog can hold 1 regiment of up to 1000 soldiers. If you combine 5 cogs into a single fleet you can transport an army of 5 combined regiments.
As for trading, you send merchants to a Centre of Trade where they give you some of the value of that trade centre multiplied by your country's trade efficiency. If you have 5 merchants in a CoT worth 500 ducats with Trade efficiency of 20% you'll get (5/20)*500*0.2 which is 20 ducats a year into your research (which must be minted if you want to use it as spendable money at the cost of inflation.
Asking questions about specific game mechanic here will help too. For example, to get troops onto boats you will need to build transport ships (cogs, in the early years). 1 cog can hold 1 regiment of up to 1000 soldiers. If you combine 5 cogs into a single fleet you can transport an army of 5 combined regiments.
As for trading, you send merchants to a Centre of Trade where they give you some of the value of that trade centre multiplied by your country's trade efficiency. If you have 5 merchants in a CoT worth 500 ducats with Trade efficiency of 20% you'll get (5/20)*500*0.2 which is 20 ducats a year into your research (which must be minted if you want to use it as spendable money at the cost of inflation.