Hi @Jugglingwiththebestofthem,
I've just done my first self assessment tax return in January and it really was surprisingly easy. I had been terrified of it since I registered as self employed in March 2018 and the relief was amazing.
If you register now you will not have to fill out a tax return until Jan 2022. That is when you will fill out your 2020 tax return.
Once I had registered I kept a little notebook and logged every payment from every client in the front. At the end of each month I logged that month's total earnings figure in the back. I skipped a few pages at the front so that I could have space for any notes I thought of later, and I put one of those neon tab markers at the top of the first page for April. Logged all April's earnings, recorded in the back, new tab and section for May, log in the back at the end of the month, repeat all year until the end of Jan 2019.
Then I repeated again throughout 2019.
I think around October 2019 I started to receive emails from HMRC reminding me to do a tax return, fully completed, by January 31st 2020. Also several emails with links to helpful video tutorials, help community, tips etc. I didn't look at any of those, just became more terrified and tried not to think about it.
Around the 10th of Jan I had one day fee to myself. I logged in, went through it all, completed in about 4 hours (I took my time to read everything and Google anything I wasn't sure of). A fair few smoke and brew breaks too 
It really was very straightforward. Questions about other incomes like pensions, property, shares etc (if I recall correctly, along those lines anyway). Having nothing meant I clicked no to most questions. And nearlly all of them were select to click yes/no so very easy.
It got to a point of asking about expenses but also gave me the choice of selecting to have a personal allowance of £1000 tax exempt. I went for that as my main expenses were car/fuel costs and when I calculated my milage I would have had about £650 deductable and the £1000 seemed a better financial option (I think I understood that bit correctly).
I'm a self emplyed cleaner and my only outlay was about £30 of products, cloths, mop & bucket etc. I hardly used any of them as my clients prefered their own products! Still got the two buckets pretty much full of stuff sitting in my porch
If you do want to claim tax relief on anything you buy then make sure to keep the receipts. If you earn £1000 a month but spend £100 on products or equipment then you will only be calculated tax on £900 (basic view of how it works). You still declare the £1000 in your total end of year figure but by then you might have 12 months of £100 expenses to be 'tax deductable - so they don't include your 'expenses' when working out how much tax you pay.
The self assessment got to the the point of asking my income for that year, I entered the figure from the back of the book, a couple more questions and then it told me how much I had to pay. I was below the tax threshold so nothing to pay on that but did get a £350 National Insurance contributions bill. You get a reference for this and it has to be paid to the HMRC office stated by Jan 31st .
All done!
If you are going to go self employed with this computer based work then I believe you can claim tax relief on a certain percentage of your home bills like electric, heating, equipment, and maybe a bit on rent and/or mortgage. I'm not too sure yet as I'm new to this myself but will look into it at some point.
I hope this is helpful and also that it puts your mind to rest a bit. Good luck 