Virus???

Every time i press a button on a website, a new tab automatically loads up advertising mac cleaner etc or other types of advertisements. its never done this before up until a couple days ago and its starting to get annoying. i could be streaming online and ads will pop up out of nowhere.

At one point a tab came up saying something along the lines of 'the tech support has detected a virus but failed to stop it' and provided me with an american number to call to sort out the issues. i decided to give it a shot and rang up. a lady with poor english was directing me on what to do and i was struggling to understand her which led to a full hour conversation of just me clicking an icon on my laptop and editing a link so that they can detect my MacBooks current issues. it then came up with over 30,000 something. i can't remember. but she said 10,000 is classed as minor but mine seems to have crashed because of so many problems. after what felt like another lifetime, she then told me if i wanted to get it fixed i could choose to pay for a lifetime warranty of roughly £400 or 2 year warranty of roughly £300.

When i told the lady i wasn't sure about this and needed to put money in my account in that case, she was persistent for me to make payment straight away. when i refused saying i don't have sufficient funds in my account to do so, she said 'use your credit card or debit card allows overdraft' but i told her i don't have a credit card nor overdraft allowance. she then transferred me to her supervisor who then also tried to persuade me to pay on the spot. i told him the same excuse and he said he will give me 2 hours to put the money in my account and that he will call me again to take payment.

i dont trust him. but i need my macbook sorted. someone help? i work 6 days a week so finding it hard to get into the city centre apple store during opening hours. if someone can give me a step to step guidance on here would be great

thank you