I want to create a new AD with a new domain and new IP address for a small coporate environment.
What are the best steps to choosing an IP address and creating a new AD with 1 single site?

Will the site host anything for external access?

A small corporate environment, say up to 150 machines, with no need for external to internal support (say hosting a website) can run on the C spec IP address range (192.0.0.0/24 and up) with a normal subnet of 255.255.255.0.

AD will need to have a Windows Server based DHCP in order to authenticate the DHCP addressed properly. You can run it on the same box or separate boxes. It will also need DNS and possibly WINS as well.

An AD domain will need a FQDN or Fully Qualified Domain Name. If you plan on in the future hosting a website then make sure you own the name that you are going to set the FQDN to (like MySite.com). If not use something like MySite.local as the FQDN.

DNS will need to be forwarded correctly through your ISP's DNS. It's possible those will change too. Make sure that you are using switches and gateway's properly. You know the ones that don't give out IP address's via it's own DHCP. And if you do use those consumer based switches then make sure to turn DHCP off on it.

Also, make sure you have a proper firewall in place between the internet and your switches. Either a server based firewall or a internet appliance.

You will need to start with your basic network design first. How many nodes (computers, servers, printers, network gear, time clocks, Ip phones, etc.) will there be? Are you connecting to the Inernet and how? What is the physical and political layout of the company? Single location? Part of a larger coporate entity? Type of business? Number of departments? All of these things play a part in your design of the AD infrastructure.

Some of the answers above would determine what TCP/IP Network ID to assign to your infrastructure. Also, you will need to name your Domain and provide a DNS domain name that is different from your external DNS domain name. For example if your Internet domain name is acme.com, then your internal domain name could be acme.net. The foundation of AD is DNS so this is important.

After you have decided your IP addressing to use, you will then need to figure out how you want to organize it... for example:

Network ID: 172.16.x.x
Router - 172.16.1.1
Network gear 172.16.2.x
Time Clocks - 172.16.3.x
Phones - 172.16.4.x
Servers - 172.16.5.x
Printers - 172.16.6.x
DHCP Scope - 172.16.7.1 to 172.16.8.254

Then, the next questions are how will the data be organized and what security is applied? Is this a publicly traded company? What Sarbanes-Oxley rules have to be considered? Does this company have any government contracts? Is it a legal or medical firm that requires additional governance by HIPPA?

How will communications be handled? Are you going to use email? Will it be external or internal email? What email server will you use?

Review some of the Microsoft documentation as well: Active Directory Best Practices

My Background: 16 years as a computer professional and 7 years with AD. Designed and implemented Active Directory and Exchange infrastructure for a multinational corporation.