Im pretty sure I made a post like this a while back, so Im gonna give you my experience for starting out
Where to start?:
Pick a nation where there is 1) A good domestic market, 2) Room for competition, 3)A way to bridge connections (East-West, North-South etc) (I talk about connections down a bit)
If I was starting out, I would look at these countries: Argentina, Iran, Peru, they are usually good starting places I have found, but again just remember the points above and search around for a .. competitive market.
Remember airline sim people only care about Quality and Connections, but will only take reasonable connections, (I believe it is something like 1.5 the distance of their intended route, one of the more experienced players may add to that, as Im not sure)
A nice way to think about it, is you want to be like Emirates rather than Norweign. Norwegian are "cheap" and fly point to point, whereas Emirates are fancy and use hub and spoke, offering loads of connections and lovely service.
Okay, so what aircraft to buy?:
This is extremely important, in my early days I used the Embraer ERJ family .... Big mistake, you need an airliner that is profitable.. use the aircraft type evaluation feature. Remember its easier to fill smaller aircraft, Id suggest the Superjet family straight away, they are relatively cheap, as efficient as the C-series or any Embraer. The only issue is that there is only one variant, the C-series has 2 variants. So depending on your circumstances, the C-series may be a better option. The Embraer E-jets are great jets, but I personally prefer the Superjets and the C-series. (Avoid the ERJ's, and all of the small seater aircraft). Stick to the regional jets for now and the C-series. Remember it's nice to maintain 3 or 4 aircraft families for maintenance reasons, but that isnt the be all and end all. I'd avoid turbo-probs where possible, some others may have different views on them, I could justify the older small ones if they were at a good price, but in the newer game worlds, I wouldn't by the new ATR's or Bombardiers. Not worth it. Again, make the "Aircraft type evaluation tool" your best friend.
Write down the routes you have in mind, and then use the aircraft type evaluation to compare Superjet, E-jet, C-series and maybe the CRJ's.
Yes, don't worry too much about how it will decrease your capacity on your aircraft, bump up the economy and business class( Don't bother with First) To get higher bars. You can up the price by 40% of default too.
Yes you need departure waves, every plane from the East lands at 10:59, and they Leave for the West at 12:00. Then they organize the return. So that the people coming from the East can connect to the West and vice versa.
Just swap to African Maintenance, I wouldn't bother considering the others in my opinion
If yu have any more specfic questions, feel free to PM :)