Hey man, welcome to Modern!

I'm not too sure how big a place Devotion really has as a mechanic in Modern, but it does exist (usually in conjunction with elvish ramp). Let me walk you through the basics.

The ramp:

Nykthos, Shrine to Nyx - This is a major part of your ramp potential. If monogreen devotion is your thing, I would include a set. They aren't even too expensive either, so yay!

Ramp combo 1: Arbor Elf + Utopia Sprawl Why this combo? The enchantment not only gives you more mana, but it counts as 1 for devotion purposes, woot!

Ramp combo 2: Arbor Elf + Garruk Wildspeaker + Utopia Sprawl Having this combo in your opening hand can allow you to cast Garruk for free on T2! (T1, arbor, T2, second land, use second land to slap Utopia on first forest, double tap with Arbor, cast Garruk and untap both.)

Ramp combo 3: Arbor Elf + Elvish Archdruid + Elvish Visionary +any other elf. Fairly self-explanatory.

Now, what to do with all this mana? Well, you could summon an Emrakul, the Aeons Torn on T3-4, or use Khalni Hydra to beat down your opponent while throwing your devotion through the roof, or cast Genesis Wave or Chord of Calling .

Here's a good primer for you to look through:

Whoops, I linked that wrong. Here: Primer

Strangleroot Geist could be good in here, he could replace Kalonian Tusker .