you should begin by planning your new infrastructure. part of the planning will including having an in depth knowledge of not only your network, but the business goals and needs. you will need to have physical and logical network diagrams to help you design and implement the new directory. The more information you have ahead of time, the better your implementation will be.
Setting up AD is relatively very easy. The hard part is getting it right the first time.
you should definitely buy some books to assist you with the concepts.
Also, design the directory with one goal in mind --> Single forest, single domain model. Then if necessary, build it out. Keep the design as simple as possible. it will make the maintenance of the directory more cost effective.
After you implement, you should also deploy tools that will help you manage AD such as SMS for app deployment, MOM for monitoring, etc...
You will see that it may take you months in properly planning and designing where the actual deployment could be done in an afternoon.