Once you have Ubuntu or Linux Mint downloaded you will need to use a program to make the .iso image of the os bootable.Also when I get the usb stick. So I just connect it with the computer and when I download Ubuntu or L.mint it will give me a option to put it in my usb stick right?
If your using a Windows machine you can use Rufus to make the usb stick bootable.
Once you have the usb ready to go; go into your BIOS > Boot > and set your machine to boot to usb. In other words make usb the first option in the boot menu. Save the changes and exit.
Once you make that change your machine should boot into the usb.