There are multiple things to consider while filing forms, including the eligibility requirements, f0rm fees and waivers, documentation, and the recipient office.
The first thing to consider is the version of the form you are using. USCIS wants you to use the latest version, which you can find and download from here.
One of the options you will see on the forms page is to show applications available to file online. Multiple forms are available for online filing, including, but not limited to, the I-90, I-130, N-400, N-565, and N-600.
The USCIS website also has a calculator that enables you to calculate flat and variable fees. You select your form from a dropdown menu, and then further select options if required.
There is also an early filing calculator for the N-400. You can submit the form early if you’ve been a resident for five years or more, or if you’ve been a permanent resident for three years or more and are married to a citizen.