First you will need to find and download a distribution of linux, I recomend either Ubuntu or Lubuntu you can safely assume you need the amd64 version if you have a modern computer. Then you will need a disk imager or burner like Etcher. Finally, you will need to get a thumb drive, it will need to have at least 8gb of space or more, otherwise it will not work. Remember: when you burn or flash a disk image to a thumb drive you will destroy any data on it!
Flashing The Thumb Drive
Remove any thumb drives that you are not going to use. Once you have downloaded etcher and your distribution of linux install etcher and then run it. Then click on "Select Image" and find the linux distribution you have downloaded, the file will usually have a picture of a disk on it. Click on "Select Drive" then select your flash drive. WHen you are ready you can press the "Flash!" button, this will take a while so get a snack and wait.
If etcher says that there is not enough disk space then you will need to get a bigger flash drive, if it says validation failed or another error restart the proccess from the beginning.
Booting From The Drive
To boot from the flash drive you will need to turn off your computer, then turn it back on and access the BIOS. For most computers you can press the "F12" button on the keyboard, if you are unsure just google how to boot to bios YOUR COMPUTER MODEL. Then when you enter the BIOS you will see an option that says something like "boot from usb" or "boot from flash drive." Remember, this varies accross different computers. Once you select the option that will allow you to boot from your flash drive your distro will boot up to an installer boot menu, select "Run Ubuntu from this USB" or "Run Lubuntu from this USB" or a similiar option if you chose a different distribution.