Which tenses use when we have two verbs in one sentence?
Eg: I want to drink water.

Ben su içermek istiyorum.

Is it correct?

I need explanation, how use and what is rule.
Thanks. 

iç-mek means to drink but içer-mek means to include. there is no a rule for these two verbs because they have different meanings.

and your sentence is wrong, right one is;

ben süt içmek istiyorum

Thanks, but which tense use? 

it depends on which tense you will use. for example;

ben süt iç-eceğ-i-m - i will drink milk (future)

ben süt iç-e-r-i-m - i drink milk (simple present tense)

ben süt iç-i-yor-u-m - i am drinking milk (present continouse)

ben süt iç-ti-m - i drunk milk (past tense)

ben süt iç-i-yor-du-m - i was drinking milk (past continouse tense)

içer-mek; as i said before, this is one is a different verb and it means "to include" in english.

With istemek it´s easy, just use infinitive: 

koşmak istiyorsun - you want to run

After that it´s not so easy.

In English it´s difficult as well. You can say "I started to run" but you can´t say "I finished to run".

You need to identify the verb which you will conjugate regularly.

The 2nd verb becomes a noun and comes before the verb.

Usually you use -dik or -me. But myself, I don´t know when to use which.

Here´s a good link: http://www.turkishclass.com/forumTitle_52775 

In addition, you have to be aware of which person you need to add as a suffix.

And often the verb will require you to add another suffix to the verb that became a noun.

Here´s an example with -dik:

Don´t believe what Tom said.

Tom´un söylediğine inanma.

söyle + diğ + i + ne

say + dik + he/she + add (n)e with inmak

Here´s an example with -me

I will go swimming.

Yüzmeye gideceğim.

Yüz + me + ye

Swim + me + (y)e with gitmek

Thanks.