Compared to office or retail, apartment buildings have a lot more day-to-day management and issues. The customer pays for all upgrades and maintenance of the property.
There are 7 steps to buying an apartment building. Each step has its own process and challenges.
You might start simply by looking on Loopnet or the MLS and running your own pro forma numbers when shopping properties.
Depending on what size of the apartment building you are looking at, this could be in the form of a Letter of Intent (LOI), or it could be a board realtor contract.