Ganas

No tengo ganas = I don't feel like doing that. I'm not in the mood, not interested.

The noun ganas means something like deseo, desire. Let's look at some common expressions with ganas.

Tengo ganas

You can use tener ganas to express your interest — or lack of interest — in doing something. No tengo ganas. I don't feel like it. I'm not up for it. I'm just not in the mood. Ya tengo ganas de estar ahí. I really want to be there, I can't wait.

Note that the complement does not have to be a verb: tengo ganas de tacos = I feel like tacos. I'm in the mood for having tacos. Tengo ganas de ti is ambiguous, ranging from a simple expression of affection to a sexual come-on.

Se me da la gana

Se me da la gana expresses the idea of feeling like doing something.

Note the grammar of this expression: It begins with se, followed by a pronoun referring to the involved person, for example, me, te, le, etc. I've also heard this with pegar: cuando se me pegue la gana, when the mood strikes me.

Me muero de ganas

The translation into English, I'm dying to, is direct here. Just remember that we need the pronominal particle: me, te, se, etc.

Ganas no me faltan

Here is a handy way of prefacing the rejection of an invitation.

Quedarse con las ganas

Quedarse con las ganas conveys the idea of not having your needs fulfilled.

There's a related construction with dejar: ¿Me vas a dejar con las ganas? You're just going to leave me here like this (with me wanting more)? I tend to associate this with blue balls, sexual frustration, but maybe that's just me.

Echar ganas

The basic idea of echar ganas is make an effort.

You can think of le in these last two examples as referring to the situation at hand.