How to register a trademark + costs?

I would like to ask for clarifications regarding different aspects of trademark registration for our business:

What exactly do we have to do to register a trademark? Is it necessary to register a trademark in multiple countries? Is it necessary to hire a professional or is this something the Intellectual Property Office helps with? How much would it generally cost?

We develop personalized natural UI technology. We are based in the UK, but naturally our marketing efforts are not really geographically limited. Taking this into account, what would be our best strategy regarding the questions above?