SEMI-URGENT) I’m 19, Going to run my own full time income business. From UK but know 0% on selfemplo

I too sell, but on a website.. If you are below the threshold all you really need to do is let HMRC know your intentions and fill in some forms.. Well that's what I was told 10 years ago and that's what I did and all was OK and nothing has happened to me YET ..But things may have changed these days 

Andy is right that you only need to tell HMRC you're going to be working self employed and then submit a tax return every year. Which am accountant can do for you if it's too scary. 

But it does seem like other processes and systems surrounding it all might also help, and a chat with a business coach might help with that. I would look very carefully into anyone you use though, as there are some charlatans about. You might actually find that a good VA would be more helpful (Virtual Assistant).

I know lots if you'd like any details and have one that I use myself. But join a local networking group on Facebook perhaps and get some recommendations? 

But you don't need to make it too complicated, you probably just need to tell HMRC, sign up to a good cloud accounting system and remember to track expenditure and income for your business on there. 

Genuinely admire your enthusiasm, but I'm hearing running before walking?

Your choices really are down to time and cash, ie how much you have spare of either.

If you want a full crash course in business, then see a business coach. Here's mine, Andy Hemming you can see if there is an Action Coach nearer to you. It won't be cheap, but your head will be spinning for the next 10 years and your business will grow.

Likewise, there is James from Akcela on the forum who I'm sure would be happy to help.

If cash is an issue but you are time rich. Then get reading and learning. You are surrounded by opportunity and resources. You're on this forum, join others, join networking groups. Just Google business support groups, they are everywhere.

If you really want to go for it and a business coach is out of your league at the moment, then get educated. Go to college, study online, but do a business course and learn the fundamentals. 

I went from lorry driver to business owner with no concept or understanding of how it all worked. I have no idea how I managed to stay in business looking back at the mistakes, missed opportunities and lack of business nonce. A huge dose of luck and then a massive catchup learning exercise with a coach which I should have done years earlier  

There has never been a better time to start a business. You can run an international drop shipping business from your smart phone. You can design and have products created in volume without owning a factory. The progress of technology and innovation is incredible, along with the sheer volume of help out there to support you. But get educated, read lots.

Try this thread, and read the E Myth. Timeless classic that lays out the structure of business and systems.
Business Books and Inspiration - Recommendations

Best of luck, let the roller coaster begin

Lot's of good advice on this thread already. I'd just like to add a list of the most basic steps you should take.

Firstly, make a business plan and decide what exactly you're offering, why people should buy it and who those people would be. Then it is easier to figure out the promotional side, as well as the finances. 

Secondly, look up the self-employment tax rules on the HRMC site. You need to register for self-assessment.

Thirdly, check what kind of insurance would be applicable to you. There are some good guides online about this, e.g. this guide on public liability insurance. It's easy to make mistakes when you're just finding your feet, so make sure you protect yourself.

Fourthly, read lots and lots of materials online!

Best of luck!