The first step is to find out which platform you are running on. If you are unsure, you are probably running amd64.
Desktops, Laptops, and Servers
The best documentation for installation for desktops, laptops, and servers is the FreeBSD Handbook:
Most people will be able to use FreeBSD without customizing the kernel. However, the Handbook also has instructions on how to build your own kernel. There is an introduction that explains why you might wish to do so.
Small Systems (aka Embedded Systems)
There is some preliminary documentation for installing FreeBSD on small systems (for instance, ARM-based).
Many people will need to investigate how to build a custom kernel.
Additional Applications (packages and ports)
Almost everyone will wish to install additional applications, whether for system administration, creating web servers, software development, and so forth.
Again, from the Handbook: