Start with a swiss army knife, a victorinox cadet would be my recommendation. There are several reasons why, one of them is to get you use to carrying a knife and how to not rely on the locking mechanism because if you do that with a slipjoint knife which consists of most swiss army knives or SAK for short. It will collapse the blade into you, sounds worse than it actually is with a SAK as you typically don't have time to put a lot of pressure into it before it collapses to do serious damage. Where as with other locking mechanisms it takes more force to make the lock collapse and if you put enough of it and manage to break said lock you can do some serious damage to your finger. Typically you may do this once or twice and there may or may not be blood, I know when I did this as a kid I typically didn't draw blood and if I did it was a few drops and I learned my lesson.
The other reason is your going to be 18 and carrying a knife most people will be looking at you like your trouble if you live in an urban area in the US and they notice you are carrying a folding knife that is visible (you probably have cops called if you carried a fixed blade, even if it was legal). Where as a SAK is seen as a work knife and doesn't attract any unwanted attention typically. Trust me I know I am in my mid 20's and I encounter this every now and than when someone notices I am carrying a knife, so I tend to carry folding knives that have deep pocket clips so all that is visible is the pocket clip or if I have the option I put it in the small pocket watch pocket. The only time I don't attract attention was when I was in uniform and looked 110% professional.
The Cadet is also quite thin for a knife and you will probably forget it's even there, as well as it's being cheap and durable. I probably own somewhere around 20 or more knives by now and rotate around 5 of them as a primary EDC and I must admit I been drooling over that knife for awhile now after having handled my brothers Cadet and he put that knife through it's paces in his job and it's still going strong.
But no matter what you get I would pick up a small SAK with tweezers and toothpick on it, there is a reason to my madness on this one. Your young and when your going on a date it's a PITA if food gets stuck in your teeth on a date, that toothpick is a life saver (not to mention she may think your crazy if you are carrying a full size knife on a first date and doesn't know you). Than there is the fact you can pick up something like a victorinox rambler which has that as well as scisors, blade, small flat head and philips (comes in handy a lot to fix electronics, it saved my phone after it went swimming in a lake once), and a bottle opener for when you get old enough. And it's by far the least intimidating thing you can pull out in public if you need to cut something not to mention very useful and pairs up very well with about any "primary" EDC you are carrying to give you added versatility.