I'm at the back end of a 45 year career as a serious DIYer. Over the years I have learned a few things so here's a very short introduction:-
1) you can do far more as a DIYer than you think you can (particularly as we now have downloadable instructions and youtube for everything) so don't be afraid to have a go - the first time it might not be as tidy/neat/whatever as a pro, but next time it will be better, and the time after that much better. (and the advice above to practice stuff first with scrap pieces is good)
2) be aware of what you can't or must not do (gas and major electrics)
3) don't be afraid to "go big" on a job - e.g. it's sometimes easier to take the sink off the wall to fit a new tap than it is to try and crawl underneath it and work upside down in a cramped space
4) don't resist buying good quality tools if you will get the use out of them - they are usually nicer to use and last longer.
5) don't resist buying cheap tools that have decent reviews for one off/infrequent use. (the big
screwfix Titan SDS breaker/drills spring to mind - huge bang for the buck for occasional use)
6) on every project, buy a new tool that makes the job easier even if it's a "nice to have" rather than "essential"; you're saving loads by DIYing, so invest - over time you'll have loads of good tools.
7) and most important, always work safely - think about what could possibly go wrong whenever you start a job. e.g. for drilling in to walls and floors "where are the electrics and plumbing?" - I suspect there is nobody out there who has done serious DIY that has never hit something they didn't intend to.....