hi. is become a big problem when we decide to use the first address on PC1 or somenthing. and is it become a big problem too if we decide routers (fa0/1) ---> switch (fa0/24). what is a generaly concept in cisco use to decide these all. thanks

If I'm understanding your question you are asking what is the generally accepted method for picking ip's for certain tasks in a subnet? IE which IP is usually the first assigned to a device like a pc and which is the last? If so then the answer is yes.
Lets say that he had wanted 4 subnets for this problem and he wanted us to skip subnet 0.
Subnet 1 would be
192.168.1.64 to 192.168.1.127
The .64 is the network so we exclude that from devices
The .65 is generally used for the default gateway ip so we exclude that from devices.
The .127 is the broadcast IP so we also exlude that from devices.

So 192.168.1.66 will be the lowest IP we can assign to a device and 192.168.1.126 will be the highest. I hope this makes sense. It's much easier to understand once you've done it a few hundred times. I usually start by determining my network, default gateway, and broadcast IP's, then whatever I'm left with are valid IP's for devices on the network.

I am pretty comfortable subnetting in my head but I'm used to a certain way of getting the information on how many networks and devices are needed. This was a little tricky gleaning the information from the problem presentation and translating that into 'Oh. He wants x.x.x.x here and x.x.x.x there. It's good, when you feel a little unsure of a problem and you have to think a little harder you know you're learning. You don't get better at something sticking with what is comfortable.