Something I can actually help with (unlike advice on mtb's!)
Tend to buy my domains from http://www.fasthosts.co.uk. Cheap and its easy to redirect your domain to hosting elsewhere. Think the .co.uk domain I bought today was about £6 for 2 years. Typically .com is about double that so not expensive.
If you are not using an online service like Tumblr then you'll need hosting - which is where the files for your website live. Cheapest, reliable hosting I've found is TSO Host. I use then for my personal hosting and know 4 or 5 people that use them. Hosting there is £15 a year: https://www.tsohost.com/product/web-hosting
Had good support when I had an issue recently. UK based too.
If you use my referral you'll get another 10% off: 10offmbb
If you need proper business level hosting and support then http://3dpixel.net are awesome. At work I think we spend about £150 a month with them. The support there is great, even if you are on their basic £40 package. Again, UK based.
Hope that helps!
If you use a web designer to build you a site they should be able to sort all this out for you.
Once you have that sorted you'll need a website - but like bikes you can have one built for £500 or £5000, just depends on what you need or can afford. If you are really on a budget i've seen some OK websites built with Tumblr which is free. A bit restrictive, but might get you started.