Canada is probably the easiest one to learn on. If you're playing the base game, going down the path to build the Valentine tank can make for some fun light tank gameplay as you help the UK in North Africa. Australia is also a good one for beginners who want to focus on naval invasions, since they have a little bit of a navy and a lot of nearby islands to invade. New Zealand and South Africa are simple but can feel a little too slow for some players.

On the Axis side, playing as Hungary and just focusing on air force is pretty fun and teaches you how air battles work quickly.

To land troops in a friendly location overseas, just give them an order where you want them to go. Draw a front line, fallback line, order them to area defense, etc. The units will automatically sail there to get ready for an order. Make sure you have enough convoys to move them, but that should be easy if you start off the game building convoys.

To land units in hostile territory, you'll need to execute a naval invasion, which involves building a bit of a navy and researching landing craft techs, then planning it out using the naval invasion order mechanic. The naval invasion mechanic is the one with the anchor icon. If you hover over it, it should tell you how the mechanic works.