Quick Answer: If you are not employed by a business under PAYE and you earn money from any source (your own business, as a director, selling things from home, etc) then you need to submit a Self-Assessment Tax Return.
HMRC Guidance: https://www.gov.uk/self-assessment-tax-returns/who-must-send-a-tax-return

Quick Answer: A Tax year starts from the 6th of April and ends on the 5th of April the following year.
HMRC Guidance: https://www.gov.uk/check-income-tax-current-year

Quick Answer: Deadline is 12 months after the end of your accounting period. This can differ and you should confirm with HMRC if you are unsure when this is, or we can do this for you. However, you must pay your Corporation Tax bill nine months and one day after the end of your accounting period.
HMRC Guidance: https://www.gov.uk/company-tax-returns

Quick Answer: Different structures are suitable for different businesses. If you are uncertain, talk to us and we can help make the decision easier.