OP, good luck to you. Sound advice to focus well on Basic first. Guaranteed you will be issued all your training gear, including sharp pointy things to cut other things. No need to supply yourself. Really.
That said, a cool inexpensive knife to get now that you'll never regret having bought later would be a Ka-Bar USMC, 7" straight blade. It's been around since WW2, the real deal, makes a great all purpose camping/outdoors knife, still available in original WW2 GI leather grip with a Marine logo leather sheath or in modern waterproof Kraton handle and kydex sheath. You can find one on the Exchange here around $45-50. Or on ( not a paid BFC Dealer ) look at an Ontario SP-1, like the KaBar but nicer ergonomic grips IMO, around $35.
It's all good. Enjoy your time.