How do I set it?

I have a router with 192.168.1.1 and want one of my computers to always have 192.168.1.100 but if I type it in properties for IP Protocol I can't get Internet-connection?

What are you typing in?

You need to set your IP to be 192.168.1.100
Set your default gateway to192.168.1.1
Set your DNS server to be 192.168.1.1

I'm assuming you still have DHCP enabled on your router. you likely have the option to select what beginning IP address your router hands out as well as the maximum number of DHCP users. this by itself not including the 192.168.1.100 address should not technically prevent you from statically assigning it to your computer, but in this case the router may be too "smart" for what you're trying to do. by this I mean it may be automatically setting the subnet mask to only allow for a smaller subnet.

but first things first, after you have properly configured your computer as described by oldspeedy, check to see if you can ping 192.168.1.1

if not, verify that your subnet mask on your router is 255.255.255.0

if that doesn't fix the problem, try setting the maximum number of DHCP users to at least 100 (but less than 254).

if after all of that you still cannot connect or ping 192.168.1.1, try disabling DHCP