Give that old thread you have a "yank" test. If it breaks - toss it.
Connecting Threads thru Amazon has a "Salt n Pepper" black -white- grey assortment that will serve your needs and they bring it to your house and you don't have to fight with the rattlesnakes. My Wal-Mart is sold out of everything too so there are several threads on the board about great places online to buy fabric.
Start small & simple. Nothing breeds success like success. Think bricks or squares. There is so much to learn with this hobby that its easy to be overwhelmed on your first project.
Last words- Finished is better than perfect.