I don't remember what episode it was it was one of the early ones. But he gets bitten by a hollow (they are basically demons of the underworld) and when you are devoured or bitten you get possesed. When that happens the hollow posses him but he ends up fighting out of it. In later episodes Ichigo uses the power of the hollow inside him to fight. You have to see the anime to find out.
Second Answer:If we're talking about when Ichigo becomes possessed by his hollow form when he doesn't know how to control it, I think it's episode 60, when he fights Byakuya Kuchki.
Actually, in Movie THREE, he was possessed by his father's sword.
If you're talking about the silly one where he becomes all seductive, that's episode 164, I believe.
Do you mean turn into a full demon? If that is the case it is movie 2!
There is a skit were kagome is controlled by someone (not episode 63 or around there) and sends an arrow through his lungs. Her reincarnation put him in a coma for fifty years.
Episode 60- 64
1st Movie!!
Episode 271
Ichigo's hollow doesn't shed tears at all.
i think 112 or 113
Bleach 158 while fighting with the Privaron Espada Gantenbainne Mosqueda
grimmjow
Ichigo first uses his Bankai in episode 29 of Bleach, titled "The 2nd Division Sorties! Ichigo is Surrounded."
Episode 271
123 and 124 I believe
Episode 47: The Avengers
it is bleach episode 54 named "day of execution"
episode 116 part 2/3
The episode is 41 -- Part 1.
Ichigo's hollow doesn't shed tears at all.
235 I believe
141 they almost kiss i am pretty sure this is the episode
bleach ost ceremony commences
i think 112 or 113