1. You need to apply for a Tier 4 Student (General) visa to get you the right to stay in the UK and pursue your course of study. Now you have been granted permission to study, you need to have a visa which legalises your stay in the UK to allow you to study here. Otherwise you constitute an "overstayer" and are liable to deportation. So, I would suggest your seek to apply for a Tier 4 student visa.
2. Here is a link to the Government website which sets out the requirements to obtain a Tier 4 Student visa https://www.gov.uk/tier-4-general-visa/overview. You can apply from within the UK and a decision will be made within 3 weeks. This will then legalise your stay in the UK. Be aware that your time spent as a student in the UK will count towards the 10 year route for obtaining naturalisation and settlement in the UK.
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