Well, essentially I use 4 things.

1. Notepad, Word, some other way to document.
2. A multi tab browser with bookmarking.
3. Google Search
4. Archiving websites.
5. A way to Screenshot pages, or Download videos.

Notepad |

• Usually I try to break my search up as I go. Targets name at the top, add all social media into 1 section, then schools, jobs, family.
• I add links to my document even if I've bookmarked, downloaded, or screenshotted. It's just a backup and is easily transferred to others.

MultiTab browser /w bookmarking |

• Every browser does this now I guess, but being able to open up dozens of tabs is necessary so as to compare information as you search. (I use multiple monitors and windows in each too but this isn't a must imo.)
• I wait until I have a full window of many many tabs, create a specific folder based on the topic/target, then save all the tabs at once to that folder on the subject. Some may do as they go but I find mass bookmarking works better for me.

Google | I only use a few minor things.

• "quoting" | quote exactly what you want to search for if two or more words.
• site: | add site:https://www.thewebsitehere.whatever (and anything specific to search on that site like .pdf, all pdf's from that one site will come up.
• Cached view | When looking for info and maybe a link no longer works, many results have a little down facing arrow, click it and click cached and it will take you to a saved version, sometimes showing you useful info.
• minus - sign | When searching "ace pilot" maybe a certain pilot, lets say george comes up too much and isn't useful, next time search "ace pilot" -george and it will discard any result with george. Useful when too many results are appearing.

Archiving websites |

• https://archive.org/ and https://archive.vn/ are the 2 I use.
• Get used to archiving to these when you find something very useful, archiving everything is slow and tedious so I don't recommend that.
• PDF's are usable/downloadable at archive.org, so I use this for those.
• Twitter posts and Linkedin tend to save better at archive.vn

Screenshot, Downloads |

• I have an add on for my browser that allows me to Screenshot the entire page quickly and easily then download the file. Each browser has their own extensions that are useful so it should be difficult to find one.
• I use https://yt1s.com/ for youtube video downloads, doesn't always work but I have other ways as well.
• I use https://www.downloadtwittervideo.com/ for twitter videos.
• I use https://ingramer.com/downloader/instagram/video/ for instagram
• https://www.getfvid.com/
• I also have an extension that can download videos from random sites, it's hit or miss.

The main thing is, practice. The more you do it, the better and faster you get at it. You'll recognize patterns in finding information and in how people use the net.

There is tons more, background checkers, domain searching, reversing phone and addresses, searching for political donations, lawsuits, business entity searching, non profit records, connections to organizations, lobbyist directory's and on and on... But ya, just start with the basics, and you'll get better over time, if you find it fun. If it's boring, like anything else you'll find reasons to quit.

Good luck.