I don't know where to start?
For starter you should read the USPTO patent applicant checklist on filing patent and its basic requirements and gather information listed therein. LINK
what if inventor or applicant is not from the USA?
Most of the countries prohibit direct filing of patent application to foreign countries. These are called security restriction. It depends on your resident country. TO better evaluate situation inventor has to check respective country ACT and Rules before filing a patent application in foreign country. PCT has listed respective sections of some countries -- following link
How to file a patent with the USPTO without a patent attorney?
You can file patent application using EFS-WEB provided you are inventor of invention.
Can be a patent filed onlne?
YES USPTO application can be filed online at EFS-WEB
Where can I get detailed information about the procedures and fees?
Detailed information are available on Fee
Detailed information on filing forms and their respective formats can be found at Link
if there is a similar patent, can I be sued from the owner?
Infringement is based on actions defined in
35 U.S.C. 271 Infringement of patent. (a) Except as otherwise provided
in this title, whoever without authority makes, uses, offers to sell,
or sells any patented invention, within the United States, or imports
into the United States any patented invention during the term of the
patent therefor, infringes the patent
Now if you are filing patent application Its not an act of infringement. But if you are filing for regulatory approval and product is patented in US its called as act of infringement.