Welcome to the forums, and the Mac world!
For your machine (and the OS), as long as you follow some sound practices, you should have no trouble with your Mac. Some of the things you need to do are:
1. Make an effort to clean off unneeded stuff from your internal hard drive often. Myself, I am doing that almost every day!
2. Don't download suspicious stuff from the internet. Two infamous programs to definitely avoid are MacKeeper, and CleanMyMac. They are real, real bad!
3. Make frequent backups to an external drive. The Mac OS does come with backup software, Time Machine, but there are also other options (not for free, but inexpensive).
4. Assuming you do not have a robust disk cleanup/maintenance/repair program like either TechTool Pro or Disk Warrior, you should run Disk Utility from the (hidden) Recovery Partition for such tasks (after doing your backup, of course!). This link describes how to boot to that partition, and what you can do with it:
https://support.apple.com/en-us/HT201314
5. If you do not have the file "Install OS X El Capitan" on your machine someplace (it is "usually" in the Applications folder), you should visit the Apple Store and download OS 10.11.4 from there. Once you do that, save it to a couple of locations (one excellent place is on an external drive).
6. Finally, keep whatever software you use up to date. That ism important.
As for the USB Flash Stick, once you insert it into one of your USB ports, launch Disk Utility. It should "see" that device. You then need to Erase and Format it. For formatting it, that depends on what you want to use the flash drive. Once you are done placing stuff on that drive, just take it out. It does not need to be "ejected".
