You need more sets of 4 for consistency and get rid of singles. Flying is green's weakness, so it's not too unusual to lose against them if you're not prepared. Crushing Vines prepares you against artifact users as well as flying, Plummet is effective if you know you're playing against a heavy flying deck, Cloudthresher is a flyer-killer, and there's many others.
Given the amount of mana-ramping creatures you have now, I would have at least 20 lands. You might get Pendelhaven or Oran-Rief, the Vastwood.
You should try to get 3 more Asceticism since it's pretty awesome for giving hexproof to all of your creatures. Harrow isn't a good card for this deck. (Rampant Growth would be better). Also, get rid of the Fog. Right now, you have too many high-cost creatures and not enough ramping.
If you want to try out other colours, borrow your friends' decks. Cheapest way to find out what you like.
I'd appreciate feedback on my elf deck. Win with infinite mana combo from Staff of Domination, alternately win with Beastmaster Ascension or Ezuri, Renegade Leader's effect, or alternately win with Hurricane. I feel like it may need more draw, or a bigger creature finisher. (And I can't afford to get Gaea's Cradle.)
For mana ramping, you probably need to choose - elves, or walls? Overgrown Battlement gets better with more walls. Minor ramping could be done with just keeping the 4 Llanowar Elves. If you go with an elf tribal deck, Priest of Titania and Elvish Archdruid would give you insane amounts of mana. Your Utopia Sprawls and Overgrowths would be better with a set of Arbor Elf, though. Greenweaver Druid is inferior to Joraga Treespeaker.
Given the amount of mana-ramping creatures you have now, I would have at least 20 lands. You might get Pendelhaven or Oran-Rief, the Vastwood.
You should try to get 3 more Asceticism since it's pretty awesome for giving hexproof to all of your creatures. Harrow isn't a good card for this deck. (Rampant Growth would be better). Also, get rid of the Fog. Right now, you have too many high-cost creatures and not enough ramping.
If you want to try out other colours, borrow your friends' decks. Cheapest way to find out what you like.
16 Forest
1 Pendelhaven
1 Treetop Village
2 Oran-Rief, the Vastwood
Creatures
4 Llanowar Elves
4 Taunting Elf
1 Joraga Treespeaker
4 Priest of Titania
2 Elvish Visionary
1 Rofellos, Llanowar Emissary
4 Elvish Archdruid
3 Ezuri, Renegade Leader
3 Masked Admirers
2 Staff of Domination
3 Beastmaster Ascension
4 Elvish Promenade
2 Hurricane
4 Asceticism