Is there a guide somewhere here for registering my business?

FiftyPents

I'm full time employed in IT, but I also do a bit of running coaching on the side. I'm looking to increase the amount of work I do in the next few months. I charge £25 a month, but also have other sources i.e weekly group sessions at a fiver each. 
I'm conscious of staying onside with the taxman, and just wanted to know how I go about setting myself up as a business - does it cost a lot of money?
Thank you!

Brynsam

See https://www.gov.uk/browse/business

You might check that your current contract with your employer allows you to set up your own business. Doesn't sound as if they are in competing fields, so with luck there's no issue regardless.

tacpot12

Have a look at this channel on YouTube: https://www.youtube.com/c/SmallBusinessToolbox/videos
I think the video called "How to become self employed in the UK" tells you what you need to know. There is a lot of useful information for small businesses on this channel.