-Dont waste gems for speeding things up or buying dragons. You will get and can breed many for free. You will need your gems for other stuff.
-Be stingy with food. Choose your best dragons for a fighting team and only feed these up for fighting in pvp league.
-Dont sell any dragons (duplicates) as long as you have enough space in your habitats. The more dragons you have, the more gold they will produce. Once your habitats are full you can sell or recall duplicates.
-Dont force levelling up. Because in the pvp league you have to fight against players that are up to 5 levels higher than you. So as long as you dont have a very strong fighting team it is better to have a low level, because winning league fights will give some gems every day.
- These are way to get gems:
Freebie island ad’s (phone)
Jewelem Tower (all devices)
Daily Shuffle (browser)
Level up (all devices)
Monday Reward (browser)
Daily Gift (browser)
League Battles (all devices)
Goals (all devices)
Earn Gems/tasks (phone)
Finishing Collections in the Dragonbook (all devices)
I think that is enough for the beginning. If you have any further questions check if there is already a thread that explains what you need. If you can not find anything just ask.