What documents to send with citizenship application?
If you would rather first read the whole DIY Guide to Naturalization first before starting, please download this ebook from Amazon.
In order to acquire American citizenship, you will definitely need to complete USCIS Form N400, and provide a whole bunch of supporting documents. As far as what to attach with your application, along with the fee, copies of your green/alien registration/permanent resident card, and pictures, you will need to provide a lot of other documents. I encourage you to use the list provided by the USCIS on its website.
The point to remember is that basically whatever you are claiming, you will need to prove it with documentation. For instance, if you are applying for naturalization on the basis of marriage to a US citizen, then you will need to provide evidence that your spouse is one, either by birth or naturalization. You will also need to provide tax returns, marriage certificate, and evidence of past marriages being dissolved.
If you have had trouble with the law, expect to provide complete documentation of that as well. If you have not been diligent about filing taxes, you will need to provide a mountain of evidence to prove what happened and what is going on now. The important thing to remember is that more is always better and if in doubt, send more, but there is no reason to overdo it.