My invention is a method to make improved products. Also, it can be used to improve the previous products by adding parts to them or replacing parts from them by improved parts. But some friends warn me from writing "previous products" so how to write the idea? Thanks
From the viewpoint of the patent law and application process, the background is necessary to define the problem that your invention solves. Therefore it is necessary to include some background, though the quantity differs largely across different applications.
The US patent law has some kind of obligation to disclose known prior art (I'm no expert for US patent law so I'm just mentioning that I read something and you should check for that).
Apart from that, you might hint the examiner what prior art there is and what he could use against you. On the other hand, especially if you do the application yourself without professional help, disclosing all you know might help you get a (possibly narrower) stronger patent, as less prior art can be used against you later if most of it was already considered during the application process.
In the end, it comes down to srategy - whatever you disclose can and will be used against you but might also help you, whatever you don't disclose may be used against you anyways.
I realize my answer might sound like an advise to disclose as much as possible - please don't understand it like that. You should differentiate from the known prior art even before the application and therefore not need the process to further narrow your claims away from prior art you already knew.