A good place to start in learning about DNA is the Wiki by the International Society of Genetic Genealogy: https://isogg.org/wiki/Genetic_genealogy
You'll learn about the different tests and why you should take each and MUCH more.
If you want to read blogs and Facebook groups on DNA, you couldn't do any better than:
Roberta Estes' blog: https://dna-explained.com/
Blaine Bettinger's blog: https://thegeneticgenealogist.com/
There are charts to help you figure out if a DNA match is a 2nd cousin, a 3rd cousin or a 2nd cousin once removed......
Two recent entries from the above listed blogs will help you with those charts:
There are Facebook groups to help adoptees find their biological parents using DNA. There are Facebook groups on GEDMATCH and the analysis tools contained there. There are Facebook groups on the DNA testing companies.
Just jump in and read, read and read some more.