Did you try Beginner's World? It's a good way to start after the Demo.
But since you're already here, here are my tips:
- Read every page of the manual.
- If you wonder about something, search the forums.
- Find an airport that isn't crowded. Remember that if you base in the EU, you can open 2nd bases in any other airport in the EU. As a beginner I wouldn't choose the biggest and most crowded airports.
- Use suitable aircraft. Some people like to run a niche airline, for instance only flying smaller turboprops, or almost purely long haul, depending on the demand out of the base you choose.
- Don't lease longer than to the next D check. Also, you might want to consider buying your aircraft after a while, so long leases aren't always that wise.
- The production lines are very backed up on popular a/c types like A320 and B737NG. MD-90, B717 and B737 classics are good aircraft that are available now. If you don't have competition on your routes, and you're not flying long haul, don't be afraid to configure your aircraft with high density seating.
- Stick to one fleet type to start with, because with every new aircraft type you get, the commonality costs increase dramatically. I wouldn't be comfy with any less than 50 of each fleet type, 30 as minimum.