If the police ask you to write a statement, which they normally do when they stop you on the road and take you to their station or when they invite you to their station, you should know how to write a good statement that will not implicate you.
First, whether you write a statement as a witness or as a suspect, you must understand that your statement is very likely going to be used against you either in court or for the purpose of blackmail by some officers (not all). Therefore, take your statement writing as a serious matter.
Remember that you have right to refuse to write a statement until your lawyer is present. But this normally leads to argument and hostile reactions from the police, and it may likely be a pretext for them not releasing you immediately.
There are no hard and fast rules as to the content of a statement. It is supposed to be what you know in relation to an incident or the reason for you being brought to the station. Since the statement is not just a general view on anything you like, it is important that you know the reason for which you have been brought to the station.
For those who have been suspected of committing an offense, make sure that before you start writing a statement, you ask the officer the offense for which you have been brought to the station. If he tells you that there is no offense, that they just want you to tell them all you know, you can politely say: No, Sir. The police force is meant to enforce the criminal law. Therefore, the police would not have any justification to bring you to the station except if they suspect that an offense has been committed somewhere by somebody - either you or someone else. And you need to know the offense and the person suspected before you could write your statement.
Also, if you were arrested or invited as a result of a petition written against you, you must demand to read carefully that petition before you start giving your statement.
When you start your statement, make sure you explain in your statement the reason for which you have been asked to make your statement. For instance, you can say something like: "I was stopped while driving my vehicle with registration number xxxxx on ________ street in _______ city. After showing my papers to officer xxxxxxx, I was taken to the police station and informed that my car was suspected of being stolen car"
Then you make your statement. You will see that the police will not like this. But you should be able to state the summary of the circumstances that led to you having to make a statement in the station.
I will say more about writing statements at police stations. For now, just get the idea that you should explain why you are making a statement and the offenses you were told you were suspected of. You should state that in your statement before you begin to write other things
POST SCRIPT: The reason you have to state in your statement the reason for making a statement is obvious. Your statement when read by a third party can only make sense in the context of the matter that prompted the statement. So, if you didn't state the matter to which your statement relates, the police may change the matter down the line. Initially, you thought you were arrested because of stolen vehicle, but the police later charged it to kidnapping. If a judge is reading your statement in a kidnapping case, he will think you were evading the issues. He will think you were dodging the matter that you don't want to answer the question. And he will form a negative opinion of you based on your presumably inconclusive and evasive statement . Be wise and be careful