I got into my role straight out of high school, working taking calls on the IT Service Desk side for a year (think call centre work, but for tech support rather than selling products) and moving up to a desktop PC support role.
I'm lucky in that my company doesn't require qualifications, just some decent customer-facing skills, enthusiasm for tech work, and a good pass mark on their technical tests. Experience is a huge plus point too, often more than qualifications, but a lot of companies out there require a qualification.
For most corporate IT roles you want a Windows Certified qualification in a relevant field -- the qualification hierarchy they use is confusing, so excuse any mistakes below.
For desktop PC support, the standard qualification is
the MCSA in Windows 7 with the focus on the Enterprise Desktop Support Technician exam (previously an MCITP or an MCDST).
For server support roles, it's the MCSE in Administering Windows Server for whatever version suits the job market you're interested in -- newest is
Server 2012 (They briefly renamed this qualification to an MCITP last year.).
The requirements vary a lot depending on the company -- my company still advertises for Server NT Administrators because a lot of their servers still run that...