We have a Ruud a/c (unit outside/furnace inside) that started leaking water last night and now there is a puddle under the unit. Need help and don't know where to look or what to do.

It could be several things, plugged condensate drain, coil may be freezing due to dirty filter or low on charge or possibly a cracked coil pan. Let me know if you have checked any of these things.

I don't know how to do those things, I'm absolutely clueless. I know it's leaking but I can't see where. There is this big round cylinder under the furnace part and water is coming either from it or from right over it. The filter was really dirty and we just changed it the night that we noticed the leaking (right before). But it acutally got worse through the night with the new filter. Could it have been that the coil was frozen and is now thawing out? We are leaving it off today while we are at work so that maybe it will dry out in there but I need more help finding the areas to check.

It could have been froze do to the dirty filter. Let us know what it does tonight when you start the system back up.