Where do I get the mana from for cards that give me mana?
You get the mana from the card itself, and you just remember that you have it for now.

I suspect you may be confusing lands with mana and thinking that this card gives you a forest card. They're different concepts completely. This is a common way for new players to get confused.

Mana is a currency that goes up and down during your turn. Lands, and creatures like Heart Warden, are ways to get this currency. Forests, for example, implicitly have exactly the same ability, and in fact that ability used to be written on the card:



Nowadays forests just have the symbol for green mana instead, but it means the same thing:



Forests give you green mana. Llanowar Elves will also give you green mana. To cast a spell that requires 3 mana, you can tap 2 forests and 1 Llanowar Elves to get 3 mana, then spend that 3 mana to cast the spell.

For more information on this, I suggest you read the Magic 2015 Quickstart Rules, page 4 on Lands.