If by tights, you mean the spandex shorts, most will have the size (in inches) on the tag. If you're a 33 inch waist, you'll probably fit a medium. Most bike shops will let you try them on (with underwear) before you buy. Not too tight to cut off circulation, but not flapping around, with enough room for your 'package'. #1 is that they should be comfortable.
For chamois (the pad on the inside), there's really a lot of choices and you should look at thickness, whether gel or not, synthetic or natural fibres, etc. You can sit on a few bikes in the shop or bring your own bike and see if they're comfortable.
I'd suggest NOT buying the lowest price ones, but maybe 2 steps up to find some decent quality in chamois and stitching.