I did my MS math in a poor math department from south Asia. While as a MS student I wrote a solo author research paper in finite group theory (without any advisor or help from anyone). I don't know any one who works in my area or who can understand my paper completely. I have no experience of writing any research paper. I have now completed my paper and want to submit it to a journal. I have no idea about submission or appropriate journal. At this point what should I do?
First question: Do you have an understanding of what is and what is not known in the specific area of your paper? If no, then you aren't done with the research yet - maybe the stuff you've proven is already known? You'll need to try and figure out what are the right keywords to put into eg Google Scholar to find the papers you need to read first. If you've completed this step, proceed to the second question.
Second question: In which journals were the papers you've read that seemed similar to yours in topic and how groundbraking/impressive/of general interest they are? These would be the natural candidates for you to submit your article to. Have a look at what these journals have recently published. Would your article fit in there? [At this stage, you'll also want to reconsider the presentation of your paper. You'll want to adapt your writing style to the target audience.]
Once you have settled on the journal you want to try, simply look at its website for the instructions to authors and follow them. Some journals will already want you to use the journal style for submissions, other's don't care. You'll then either upload the pdf of your article on an online submission system, or sometimes email it to the relevant editor. To be clear, the pdf you are submitting should be created with LaTeX and follow standard formatting conventions whether or not the journal specifies a particular style.