Just some advices to you (I am playing AM since last round and those may not be perfect but may help you a bit. Ok, I am No.1 in Central America :wink: ). :
At THIS stage of the game try to find airports worth 200k/250k a gate (have not checked if there are still some available).
Connect ALL airports (maybe not those <50,000 pax) that are close to this airport to your base.
Check competition on the routes BEFORE renting a gate. It does not make sense to rent a gate e.g. at Heathrow as the competition will probably have ALL european airports <200k connected to Heathrow and may run them on low fares already which means you will not get any profit from those routes.
Do not even think of starting long haul operations until you have no airports close to your base connected to it.
When you have around 5-6 gates at you base filled up with flights look for a second base. This one should not be too far away from your first one. Then you will be able to create new routes to the destionations you have chosen from your first hub. Re-using the gates is a good way to save money instead of spending more on new gates.
Last but not least about aircrafts:
Do NOT save the money three real days long doing nothing to order a Boeing 737. In those days you could have bought a lot smaller aircrafts which will give you a higher profit than a single 737.
Also, do NOT lease in a B737 or a bigger one. I am sure you will spend more money on running it then you will receive from it. I still have some smaller aircrafts flying around giving me not the highest profit but still enough that I am not considering to replace them at the moment. The aircrafts I buy I need for creating NEW routes.
The good thing for you right now is that many people get rid of their older and smaller aircraft. Take a look at the used market. There are some cheap aircrafts to buy.
Remember: the above given advices are just for your specific case starting late in this round. At the beginning of the next round you may have to change parts of the strategy. Feel free to drop me a PM if you need some more help.