Get a free Spotify account and listen to the different recordings. Read reviews if you feel inclined. For Mozart's Requiem, you have to choose between recordings on modern instruments or period instruments, so it comes down to personal preference. On modern instruments usually the most recommended CDs are this, this, and this. Have fun finding something you like.

I highly recommend this album, conducted by Philippe Herreweghe and played on period instruments. It's my favorite recording of the Requiem, and one of my favorite albums overall. It just sucks me in from the first bar.

However, I would suggest checking out several recordings and seeing what suits you best. The Gardiner, Solti and Marriner recordings all have great aspects to them and are very highly regarded.