I think

[ROBINDESBOIS]

Which sentences are correct?

I think is necessary
I thik it necessary
I think it is necessary
I think it is necessary to have a positive attitude in life
Having a positive attitude in life
have a possitive attitude in life

[Perikles]

Which sentences are correct?

I think is necessary:bad:
I thik it necessary....:bad::good:(I think it necessary to dress smartly):good:
I think it is necessary:good:
I think it is necessary to have a positive attitude in life:good:(to life):good:
Having a positive attitude in life:good:(not a sentence, it's a phrase)
have a possitive attitude in life:bad:I think it necessary to have a positive attitude in life:good:
have a positive attitude in life! (correct if an imperative)

[wrholt]

I agree with Perikles assessment, at least from the US perspective. Howver, to my ear "I think it is necessary to dress smartly" sounds a little better than "I think it necessary to dress smartly".

[Perikles]

I agree with Perikles assessment, at least from the US perspective. Howver, to my ear "I think it is necessary to dress smartly" sounds a little better than "I think it necessary to dress smartly".I agree with you.

[ROBINDESBOIS]

I think it necessary to have a positive attitude in life:good:
have a positive attitude in life! (correct if an imperative)

I think it is necessary to have a positive attitude in life
I think it is necessary having a positive attitude in life
I think it is necessary have a positive attitude in life.

Are they all correct?

[Perikles]

I think it is necessary to have a positive attitude in life:good:
I think it is necessary having a positive attitude in life:bad:
I think it is necessary have a positive attitude in life.:bad:

Are they all correct?no.

It is necessary to have a full infinitive after 'it is necessary..' :D

You could always say

I think (that) having a positive attitude in life is necessary :good:

here, 'having a positive attitude in life' is a noun phrase