Hey there. You would take your telescope focal length divided by your selected eyepiece's focal length. Your etx 90's focal length is 1250mm. So if you were using a 26mm eye piece. it would be 1250/26 = 73X If you use a barlow lense you would multiply your focal result by 2. ie. 73 x 2 =146x.

For eye pieces, I would suggest having 26mm, 18mm, 9mm, and a 5mm.. Those are good to start with. For viewing the moon it's good to start with a 26mm to find your target view are, then work your way down in mm lenses for more specific areas or parts of your viewing object, and adjust your focuser accordingly. I have an etx 90AT.. I haven't looked at Saturn yet, but I would imagine that an 18mm would be a good start, for actual detail viewing I would suggest a 5mm lense, and make sure you are using a barlow eyepiece with all of these.