Buy the best you can afford
You get what you pay for, and some brands or models deserve their reputation. This is true in leather craft also; "buy cheap, buy twice" goes the old adage. I started with the cheap starter kit for $100 and later upgraded to more expensive high quality tools. I can't even tell you how many times I destroyed a good piece of leather just because of a cheap beveler.
Some things you can save on, are an expensive mallet or a fancy sanding and burnishing power tools. Set your budget for getting started and allocate it proportionally to the most important tools. For me, the most essential is a knife that fits good in your hand, metal ruler and of course high quality pricking irons.
Measure twice, cut once
I'm sure we've all heard the saying "measure twice, cut once" but I cannot stress how true this is. You can only imagine how many times I ruined a big piece of leather just because of bad measurement.
This is the point where a good metal ruler comes to the scene. A Plastic ruler is a terrible idea just because your knife blade is sharp and can go through your ruler and you end up with scratched leather.
Precise measuring and safety is always priority number one!
Same goes for stitching and your thread lengths. Good rule of thumb is to measure the stitching holes lenght on your masterpiece and multiply it by 4. But this is not always the case, because it depends on factors like leather thickness and hole spacing.
Don't be too hard on yourself
One of the reasons I got into leather crafting was seeing all the creative and wonderful items posted all over the internet. Expensive shell cordovan, perfect stitching and those shiny edges!
Nothing can be perfect for the first time. You WILL make mistakes, you WILL learn from them and after some time, you WILL make a masterpiece!
*You will ruin some expensive leather pieces tho*
This is a craft. Craft skills are not innate, but learned, practiced, and honed.
Take your time, enjoy every single minute, and congratulate yourself when deserved. Just don't be too hard on yourself!
Everything is better when you can talk to someone interested in the same hobby as you. I have talked to so many awesome people on the internet just because of the interest in the same hobby, and let me tell you, you'll learn a LOT just from talking.
I would also suggest you to post any questions on Reddit because there are so many helpful people, and you will always get the best answers from more experienced crafters and beginners like you. Watching YouTube tutorials and leather crafting YouTubers helps a lot.