Get a lesson from an insured and qualified SUP Instructor and don't be afraid to ask who they are qualified and insured with. Water Skills Academy, ASI, BSUPA and ISA are some of the UK training providers also www.gbsup.co.uk have a list of schools you around the country. Sadly there are well intentioned people offering lessons and will happily take your money who aren't qualified and while SUP is a simple and safe sport it is important to learn what conditions and locations are safe for you especially while you learn. You don't want to end up being a RNLI statistic.
Don't rush out and buy the first board you see. As tempting as that is take your time (I rushed out and brought a board...then another...then another!) Try as many boards as you can you will find that you outgrow the beginner boards very quickly. As they are designed for stability they are super wide and not so suitable for longer paddling ....patience young grasshopper on your board selection you and your bank balance will thank you for it!