Pick a paper from a journal with a high impact factor to avoid doing a really crappy paper. Make sure you read through the methods part especially carefully because the others will ask questions such as "At what dosis was this performed", or "How much salt did they use for this extraction?". If you don't understand a method look it up or ask a more experienced member of your lab.
If methods are used that you yourself don't use try to find someone that does use them and ask if the experiments were done correctly. Be critical, always ask what controls are there/missing.
After you have prepared it ask a postdoc to go through with it and help you find the weaknesses of the paper.
Finally, don't take it personally if the others weren't convinced of the paper- in three years there was no paper that cnvinced everybody in my lab