You'll need to do extensive research on your country and the position it takes on various issues. Here are some tips for conducting that research.
1) Look up your country's permanent mission to the UN
2) Read speeches, resolutions, and past records
3) Learn about the background of your country, from geographical region to population
4) Look at newspaper articles to learn about current trends and stances
5) Use the library's search engine, Primo, or Google Scholar. Type in UN and [your country] and [issue] and see what appears. Play around with different keyword searches to see what produces the best results.
6) Talk to a librarian (via chat, email, phone, or in person)
This suggestion is adapted from the Best Delagate Model UN website.