how to take care of iphone battery so that it won't deteriorate fastly or it won't reduced its capacity?

Take a look at this useful user tip:--> When to charge your iPhone or iPad - Apple Community

Regards

Giulio

Just to be very clear. You cannot prevent your battery from losing capacity. Batteries are consumables. They will eventually become unusable there's very little you can do to slow that down by much.

The article ProustGiulio recommends is excellent.