Date her, that being said, you can't just say that you're just trying it out, she won't believe you. She'll only believe what she experiences, not what you say.
Imagine yourself, you're dating a girl, she says she just trying it out and asks you not to take the relationship too seriously, and after one month, everything seems to be going great, so you think you've won her over, but suddenly, she breaks up with you, and you have no idea what happened. You're bound to be very resentful if that happens to you. The same goes if the reverse happens and you break up with this girl who likes you one month from now, for apparently no reason.
So being explicit about wanting a tentative dating relationship is not really going to work. The next best thing is to be explicit about not wanting an exclusive dating relationship. This will convey the right message to her, that the dating relationship must have some clear boundaries and that she can't take the dating too seriously. Say this even if you're willing to date her exclusively and even if you have no idea if you'll be dating other girls or not.
If she doesn't want to date you under those terms, or continue dating you under those terms, that's fine, but it will be her decision.
Keep things light initially. No fancy dinners. Don't let her spend too much money on you. Split the bills, or take turns paying. Coffee, tea, picnics, lunches, home-packed lunches and home cooked dinner. Correct her privately afterward if she calls you her boyfriend publicly. But if she does it again, correct her publicly.
Don't go out with her more than two or three times a week, or she will think there is something more. Don't spend the night with her on Valentine's day (even if you're not dating anyone else). Don't let come to your place unannounced, or uninvited. Don't let her leave stuff at your place to mark her territory, clothes, a lost earring, etc.
That being said, you should not to date while going out in a group. The peer pressure and the judgments from your peers will only get in the way.
If she asks you about other girls, that you might be dating or that you might be interested in dating, or that you previously dated:
Tell her that "I don't talk about other girls".
Say this even if you're not dating anyone else. She doesn't need to know. Repeat this line as many times as necessary. And the same goes for her too, tell her that you're not interested in hearing about any guy she's dating, or interested in dating, or has previously dated (assuming she's sexually responsible).
If she becomes too possessive, end the dating relationship. Dating like this is not for everyone. It's not anybody's fault when something like that happens. It's just nature. Just don't continue dating her when you see those red flags, or her possessiveness will only get worst.
And yes, like AndreiROM says, there are tears, I've cried before when those types of relationships ended, but I wouldn't trade those past experiences for the World. And waiting until you have the perfect girl in front of you before dating her is just hogwash. If you don't have experience, you really don't know what you want in a relationship anyway.