What scene and act did Iago convince Othello to get drunk in Othello?

I also believe that you're referring to Act 2, scenes 2
and 3--the scenes in which Iago plans to get Cassio drunk so that
when Roderigo provokes Cassio, Cassio will retaliate (and thus will be fired by Othello
from the lieutenancy).

In Act 2, scene 2, Iago convinces
Roderigo (who is in love with Desdemona) that Cassio and Desdemona are having an
affair.  While this claim seems ridiculous to audiences, Roderigo, who is so desperate
to win Desdemona's love, believes it because Iago is so convincing.  Iago tells Roderigo
that the only way for Cassio to be out of the picture is for him to be dismissed from
the lieutenancy, so convinces Roderigo to provoke him and start a fight.  Further, Iago
convinces Roderigo that Cassio is "rash and very sudden in choler, and haply may strike
at you," and decides that if Cassio is drunk, the plan will be more
successful. 

Then, in an a brief soliloquy in Act 2, scene
3, Iago tells audiences of this plan to get Cassio
drunk:

If I
can fasten but one cup upon him,

With that which he hath
drunk tonight already,

He'll be as full of quarrel and
offense

As my young mistress'
dog.

Ultimately, Iago's plan
does lead to Cassio's dismissal from the lieutenancy, which is, obviously, a victory for
Iago.