Depending on how long its been living on its own, it will probably take a lot of time for it to really trust you, let alone catch it. Best way in my opinion, besides using a humane trap, is to try and lure it to you with food. Maybe set the food 10 feet from you and just sit there and see if the cat will come eat that close to you. And then move the food closer and closer to you.
As for you being allergic to cats and possibly wanting to keep it, I am also allergic to cats, but I have always had cats. I take an allergy pill (zyrtec is the only one that works for me) every day, and I never have any sort of reaction to the cats. It took a few days of it to kick in, but after that I haven't had any problems.