Thank you for taking in the cat. In the past few years we have had so many cats show up and we end up taking care of them. It is so heartwrenching that so many cats are abandoned by their owners.
A few things you should know. First, my vet told me that the frontline you get from places other than vets are fake. I have found a place that does inexpensive neuter/spay and they have the frontline at a very cheaper price. However since you are going to be caring for the cat, I would highly recommend that you take it to the vet to make sure all is well with the cat. It most likely will have worms also. If your neighbor didn't give you a rabies paper, you will need to get a rabies shot for the cat as well. This is for your protection as well as the cat.
And finally, if the cat is not spayed or neutered, please do so. If you really can't afford it, contact your local cat rescue (you can go to petfinder.com and do a search). In my area there are many cat rescue outfits that will gladly help you get your cat fixed at a low cost so that there is no more babies born.
And again, thank you for adopting the cat.