A few things I can recommend:
You have most likely done a literature review on your area of research. Take a look at where those papers have been submitted (the more recent ones especially), and search for those specific conferences online for more information. You can use google scholar to winnow by publication date and can view the number of paper citations.
A quick search online came up with these lists for top data mining conferences, which can give you a reasonable start:
Microsoft Academic Search, Conferences in Data Mining, Microsoft Academic Search, Natural language & speech, The top 10 NLP conferences.
Search within a few top conferences to see if papers on your specific topic have been published within them. Try 'text data mining proceedings' in google scholar for some ideas as well.
Don't forget to ask other graduate students who have been around longer for their advice as well. They will probably be able to tell you at least two or three top conferences that they regularly apply to each year.
I'm not sure about your field specifically, but in my experience conferences are grouped between submission dates in the fall and submission dates in the spring. Once you have a list of reasonable conferences you can choose one by the current date appropriately.