I am writing this article AS a beginner myself for other beginners.
Dont be scared to attempt anything you can think of, someone somewhere will have written a full guide to achieve it.
Talk to sellers before buying parts, its easy to spend a lot of money, and then find cheaper parts do the same or in some cases more accurate job cheaper too.
When following online tutorials read everything twice, and make sure to read the comments, many times you will find the project is for older os, or has some typo's and are pointed out within the comments, a typo can leave us beginners totally stuck, we wont spot them, and this can lead to pizza pi!
Dont expect one pi to behave like a billion dollar super computer and control the entire world, The irony is it will, it will just leave you sitting on your hands waiting for it to do everything lol. So keep pi's to a bunch of related tasks, and add extra pi's when performance drops below acceptable. For example a single pi wont run a website as big as apple, with all those users your ISP will fall over first! but you will need more than one pi for a website.
The most compatible SD cards for the pi tend to be the cheapest ones! (Tesco 16gb works incredibly well. Buy online they are so cheap.)
Your only limit is your own imagination, other forum members have helped me so many times ive got stuff running which i still dont understand lol.
And above all have fun, I know i am.