Tier 2 is available for skilled workers with a job offer. Your employer must sponsor you to apply under Tier 2, and they may do this if the position appears on the Shortage Occupation List. An application under this route is also subject to the salary of the job offered, your own personal funds (maintenance) and other requirements.
The above applies to Tier 2 (General), but there are also specific categories for Ministers of Religion, Sportspersons and Intra-company transfers. See the UKVI website for further information.
If you are currently in the UK as a Tier 4 (General) student you can apply to switch into Tier 2 after successfully completing your degree level studies. The employer will issue a Certificate of Sponsorship (CoS) if you are being sponsored to undertake a genuine vacancy and are capable of undertaking the role for which your Certificate of Sponsorship was assigned. The requirements of the job must be reasonable and not have been tailored to exclude resident workers from being recruited.
VISA can give you basic advice on switching into Tier 2, but if you are making an application, you must seek specialist advice either from your employer, or a qualified adviser.