Visitor Visa - B2

Firstly, i want to say thanks to immihelp.com and everybody who have been contributed with their experiences here, you are a such a wonderful poeple. I wish to apply for US visitor visa, pls i need your advise sir/ma, am a Firefighter from Nigeria, 33yrs old, single, my annual salary is $3,810, i want to visit my instructor (teacher) to have more knowledge about my job,for 3 months or more,he is a USA citizen, he came to Nigeria last year to come and teach us about the job with our new equipment, but we had a very limited time to complete the course, i have all the documents to show that am a Firefighter even with the pictures we took together before he left, he has sent me the invitation letter and wrote in there that he is going to responsible for my accommodation and transportation for the period of my staying with him,he attached a photocopy of his passport to it and even added his contact address and his phone number,i have a valid passport which got early this year, i haven't travel to any other country before, this is going to be a very first time. my question is what are my chance of getting visa? or is there any other thing i need to add? Pls. i would appreciate your quick response.Thanks.