http://www.mtggoldfish.com/articles/budget-commander-selvala-explorer-returned-57

How It Plays
Selvala is an open-ended commander that can be built a multitude of ways. Her ability is useful for any Green/White deck: every type of deck likes card draw, ramp, and life gain. You can slam together a bunch of generic goodstuff Green/White cards and have a good deck with her at the helm. However, focusing on a specific aspect of Selvala can yield more focused themes to build upon:

Group Hug. You're already drawing people cards with your commander, so it's not a big leap to build a deck around giving everyone more resources with cards like Howling Mine and Hunted Wumpus.
Lifegain. Selvala gains 0-4 life with her ability in a typical 4-player game. Depending on how you want to build you deck, she's an excellent alternative to Trostani, Selesnya's Voice (though the other will probably be in your 99 anyway).
Elf Tribal. Selvala is an elf! Add more elves and elves-matter cards like Priest of Titania to get an elfball rolling!
Stax. Welcome to the dark side. Since Selvala is an excellent source of mana and you're in Green for all the mana dorks you could ever want, you're in a prime position to lock down the board with devious stax cards like Winter Orb, Armageddon, and Stony Silence. Caution: Don't go this path outside of competitive playgroups or everyone will hate you.
For this primer, I'll be discussing Elf Tribal Selvala.



Elfball!
At its heart, Elf Tribal is all about the mana dorks: Elvish Mystic, Llanowar Elves, Fyndhorn Elves and others are the heart of any good Elf deck. From there we have elves that skyrocket our mana (Wirewood Channeler), pump our elves (Timberwatch Elf), make tokens (Lys Alana Huntmaster), or swing in for the win (Ezuri, Renegade Leader).

Since dumping tons of mana dorks will empty our hand quickly and Selvala is both an excellent source of card draw and mana acceleration, we're also going to be running a bunch of untap effects for some really explosive turns.

The game plan is like so:

Play mana dorks.
Drop down Selvala, Explorer Returned.
Tap Selvala + mana dorks for big mana, untap Selvala + mana dorks and retap them for big mana.
Cast giant spells to refill our hand, drop big fatties, or win the game off a lethal Hurricane.
Sounds good? Let's get started!

Elfball has tons of avenues to win. Here's a couple this deck is capable of:

Lethal burn. Burn in Green? Yup! Generate a ton of mana in one big turn via untap effects or an infinite mana combo, then kill the table with a massive Hurricane or Squall Line. Note you must have more life than your opponents to win, but Selvala, Explorer Return gives you extra life so this should be easy.
Turn your dorks into fatties! We have numerous ways to do this: Mirror Entity can make our army infinitely large, Beastmaster Ascension can do similar, Triumph of the Hordes gives us a cheat around life totals along with trample.
Big fattie smash! It ain't fancy, but sometimes bashing face with a random big creature like Soul of the Harvest will do the trick.