If you are not going to pay to use a registered patent attorney or agent you have a few options. One is do it yourself. It will take much studying and isn't really reliable. You can join a local inventors club and get some advice from other inventors. To have a reasonable chance doing it yourself, you would need to make patent law and patent office procedures an all-consuming hobby. The book patent it yourself is a good start if you were going for a simple mechanical invention. Starting with a computer implemented invention is biting off a huge task with tons of pitfalls.
The actual fees to the USPTO aren't too bad for a "small entity" if you qualify as a "micro entity" you get 75% off most fees. That would make filing a regular non-provisional application about $370. If you go through the system very smoothly the next fee due will be the issue fee of about $400 for micro entities.
The USPTO has started a pro bono program in conjunction with bar associations around the country. It might be worth looking into that. I know the program is up and running in Minnesota and California.