If you are an adult and want to undertake a short course of study in the UK (such as a beginner's English Language course or a work-related training course), you might be able to come here as a student visitor. If you want to study a longer course, or you want to work (including on a work placement) alongside your studies, you should apply under Tier 4 (General) of the points-based system.
Student visitors are allowed to come to the UK for 6 months (or 11 months if they will be studying an English Language course). When you enter the UK, immigration office will stamp the duration of your permission to stay in your passport. You cannot extend your stay beyond this period.