You don't pay any tax on dividends from shares within an ISA.
Outside an ISA, from 6 April 2018, you won't pay tax on your first £2,000 of dividends in the tax year. However, beyond £2,000, you pay tax depending on your income tax band. Basic rate tax-payers pay 7.5%, higher rate tax-payers pay 32.5%, and additional rate tax payers pay 38.1%.
Any gains made by investments within your ISA are not subject to capital gains tax.
For most people, income and capital gains from ISAs do not have to be reported on your tax return.