To my knowledge, once u get a warning letter your not entitled to increment/bonus etc... and to some extent, this would be reflect consistently in your KPI (if applicable to you).

My thoughts/ 2 cents:
1. a person should be a given an opportunity to answer any issues arising in his/her employment;

2. as such a person would be given an opportunity to answer such issues before a panel (domestic enquiry?) or counseled by a qualified HR personnel;

3. finally, if all reasoning fails only than 1st/2nd/3rd warning letters are issued.

Unless of course an employee has been found to have committed some serious offence i.e. theft.

if i no sign it..they ask me get lost from this company...if i sign it..they easy to kick my out early???can some one adviece wat can i do?

I have to agree with you.

If you do sign the letter.
The implication is, if they choose to terminate your employment from the face of it looks pretty legitimate. Since you yourself sign off on the paper and technically agreed to all the statements contained in the warning letter. As such they don't have to pay you any compensation for immediate termination.

Example:
Normally depending on the notice period your required to give to your employer.

Say: your contract states that you are required to give 6 months notice, if your employer requires you to leave immediately. They need to compensate you with 6 months salary. This applies to the employee as well. (such terms may vary, this is just a benchmark)

If you don't sign the letter
Assuming:
1. they didn't go by the book;
2. they didn't give you an opportunity to reply; and
3. the "nature" of your so called offence is minor and does not warrant a warning letter.

Than i can safely say that you shouldn't sign off on that letter, you should:
1. ask for an explanation;
2. if the contents are untrue or malicious in nature, ask for an explanation;
3. if the contents are true to a certain extent, ask for opportunity to answer those allegations; and
4. finally try and negotiate with your employers not to issue you a warning letter (if possible).

To protect yourself, i suggest:
1. you put all correspondence in writting;
2. ask HR/Manager or whomever to put it in writing before corresponding with you (if otherwise i.e. verbal just ignore);
3. if in any doubt, put in writing.

Good luck.