You need to talk to a good criminal defense attorney, as soon as possible.  It sounds like there are either charges pending against you, or that charges could be filed.  The police don't usually make this sort of request of someone, without some leverage of that sort in their pocket.

It might make sense to cooperate with the police, to do what they ask.  But before agreeing, you need to make sure that whatever you are promised in return for what you're going to do is in writing, and can be enforced, and that's where a good lawyer comes in.