To get a true "Mentor", visit the game mentor page (under the "dashboard" heading) and request one. Your request will be sent to the pool of experienced volunteer mentors, and the first one to respond will be your official mentor. An official mentor can see behind-the-scenes of your airline, so they can help you best.
If you are just looking for general suggestions, take a look at Swiftus' and Curse's beginners' guides in the general forum. If you have any general wonderings, these can be pretty helpful.
As for your base choice, starting a full game halfway through can be tricky at larger bases (such as LGA). You are free to give it a shot, but moving might be easier. If you choose to stay, here are some tips:
1) Price wars don't work as well as you think. The most effective way of competing against an incumbent is having a higher company image (spend more on marketing) and run higher frequencies (4x Caravelle vs. 2x DC-8). Keep your prices at or just below default settings.
2) Watch delays/cancellations and aircraft condition. If you fall behind on either of these two factors, it makes it much harder to compete. A good rule of thumb is 90% delay and 85% condition on all routes/aircraft.
3) Start locally. Aircraft (with the exception of widebodies) are much more profitable at short range than long range. Pack four 300NM flights rather than three 500NM flights.