Particles を and に

You use「を」because you are doing the action of drinking the tea, here's a bit more also...

If you say 「９時はんは…」you are referring to the time in general, and something might or might not take place at that time. So you want to let it be known that you are "moving" into that time by using that 「に」particle

例えば：「９時半にお茶を飲みます」"I will drink tea at 9:30." If AM「午前」if PM「午後」so,「午前９時半にお茶を飲みます」. Also, because you are stating a plan, you can use 「つもり」= an intention; or 「予定」= plan with certainty, and that would read like this「午前９時半にお茶を飲むつもりです」"I intend to drink tea at 9:30am" or「午前９時半にお茶を飲む予定です」"I plan to drink tea at 9:30am"

9時はんにお茶を飲みます。 

If you want to say an act on a time, you have to use に:

6時におきます。

I wake up at 6 o'clock.

ごご　10時に　ねます。
I sleep at 10 p.m.

If you do something to an object, you have to use を:

ラジオを　ききます。

I listen to radio.

はさみで　かみを　きりました。

I cut the paper with scissors.

てがみを　かきました。

I wrote a letter.

*I hope you understand it. My English isn't very good.