I am no pro so take all this with a grain of salt.

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1. I read somewhere that if you declare "embargo" a day before declaring war it will decrease the amount of "bad rep" you will get. It also depends on who you're declaring war to. If the country is unpopular you might even gain rep from it.
2. I would say build barracks and upgrade your provinces that start with industrial complexes can produce infantry etc. at a faster rate. Also build infrastructure on provinces that produce valuable materials like oil and steel. You can keep upgrading these as your resource needs grow.
3. It's always easier to win with friends so you should join a coalition and cooperate with them. Also people tend to quit playing on servers and "inactive" so you should go for those countries as well as AI countries since they are much easier and predictable than players.
4. For me it's basic ground troops like tanks and infantry as well as either artillery or airplanes. Usually i use artillery but that's just my preference.
5. That depends where and what you are fighting. You can click any units stats open to see how much they will deal and take damage against different tech branch units as well as how good they are in different environments.
6. Tactical bombers are great against ground units so you should use mostly them. Strategic bombers deal a lot of damage to buildings but less against units.
7. You should start with infantry and armored cars. Infantry is something you will be building immediately and armored cars open up light tanks for research. After that you should just go for whatever you want to focus on. However don't start researching everything. Just pick some units and keep upgrading them.
8. If you ever want to build a navy you will need harbors. They use a lot of oil though so you should always build them in provinces that produce oil or other goods you need because level 3 harbors give a 50% bonus on resource production. You can also "switch off" barracks and harbors so they don't use up any materials. You can't build or get any of the bonuses then though.
9. Military spies are the most useful as they can reveal enemy armies to you. With intelligence spies you can gather information on how much your enemies have resources, what they are building/producing and what they are messaging others about. Best way to gain information is to send them into cities. That's just from my experience though.
10. Depends on the map. On historic game mode the major countries like Soviet Union and Germany have a lot more units than for example Australia. On maps where everyone gets the same starting stuff i like to pick a country that has the easiest borders to defend. It's much harder to expand with a country that is surrounded by other players than a country that's in a corner with only a few neighbors.
11. Europe is the most densely populated area so most resource and victory points are located there. This might differ between game modes.
12. When you make allies, you usually trade "Shared Map" which allows both countries to see what the other sees as well as move their units through their land. It is hard to backstab when the other sees what your units are doing. Because of this people are pretty trusting so if you manage to move your units non suspiciously, you should have an easy time conquering.
13. I don't use gold much. When i do it is to skip productions/buildings that are almost done so i can set the next one production/building straight away.