I forgot my passcode

The computer is an essential part of being able to remove the passcode to get back into your device. You are going to have to use one. That being said, you don't have to use a computer that you own, you can use someone else's computer or borrow one from someone that you know.

If you are using a PC, then you have to use one with the most current version of iTunes installed on it. If you are using a Mac, then you are going to have to locate it in "Finder" and access everything through there. You aren't going to be able to get it working again without a computer as this is going to remove and restore the OS on your iPhone to get you back into it.