Before starting a War is a good idea to consult the Foreign Affairs Advisor. If you have Jan de Witt in Congress the report will be much more helpful, give Military and naval Strengths and their current Population. This will give you a good idea of your potential enemies power, if they seem excessively strong it may not be a good idea to attack them, unless you think you can capture one of their weaker Colonies, and make them sue for peace. Of course they may not sue or offer terms that you can not accept. Sometimes it is better to attack a weaker power or wait for your military strength to build.A long War can be very costly, due to Colony squares being blockaded thus lessening resources, increased demand for muskets, horses, artillery and War ships, and difficulty in founding new Colonies, Learning from the Indians, exploring lost City rumers and doing pioneer work. A surgical strike is best, blockade a colony (land and sea) and hit it with overwhelming force then sue for peace.
If you have a lot of money you could start Wars between European Powers, to weaken both, but these usually end after a Colony is captured and cost a lot to initiate. Also the defeated soldiers will found new Colonies making it more difficult to destroy them. Indian Allies can be useful, if you are them with enough muskets and horses, but this also costs a lot and risks getting the Tribe annihilated.
Don't fight more than one power at a time. Your troops and ships will get damaged unnecessarily by the other Powers, and if you are at peace with them they may declare War on your enemy instead.
Activate the end of turn Command once you have finished the exploration phase, and start fighting battles. This will give you time between turns to access the situation, and move troops where they can be of most use. Also fortified Units can be made active. Remember once the end of turn message appears, even if you unfortify troops, they will not be available to move until next turn. There are 3 ways to get round this: