Welcome aboard! Oh, how exciting ... a new place with a lovely large yard to play in. Fun times ahead, who needs a gym when you've got a yard and garden!
I'm going to second and third a few comments. First, skip the pump rental - you are going to need a decent pump, and in fact, you will need a spare - so I would just bite the bullet and go and buy this Pondmaster pump -
Pondmaster Mag 5 570 GPH. * It is a good quality low energy pump that will last you quite a while, and you can use it while you are getting the pond up and running. Just looking at your picture and guessing at the pond size, I'm thinking something in the neighborhood of 500 - 600 GPH will get the water circulating. And circulating water is life.
Oh, and speaking of life - don't be surprised to find many live things down in the pond. You might think it's la brea moved into your own backyard, but a goldfish or two or three could be lurking away down there as snug as a bug in a rug. Dead frogs? Is it possible they jumped in and couldn't get back out? Is it still cold in DFW area? Could they still be hibernating?
Regardless of what the pond looks like, it is full of life - beneficial bacteria that folks pay lots of money in concentrated gallons to get their ponds going. Shakaho is right, I would not scrub away all that good stuff. Bucket or net out the really gunky leaves and things, and start moving the water. if you get the big stuff out, it would be fairly easy to get a temporary filter set up to start catching smaller stuff. You are on city water I assume? City water will have chemicals that will kill all the life in your pond, so you will want some place to hold water while you can treat it, temporarily. Trash cans work pretty good for this. And you can use your new pump to pump water into and out of the pond.
You'll get advise all over the place in here, but if it were me - I would start getting the gunk out, get the water moving and work on letting a temporary filter clean things up ... then you can better assess what you have to work with.
Oh, and by getting the water moving, what I have in mind is simply dropping the pump in the bottom of the pond and letting it circulate water on the bottom of the pond. Leave the waterfall connection and hookup for another time. Of course, you can't really drop a non-solids handling pump on the bottom of a pond until you get most of the big gunk off the bottom.
Another oh. A good temporary filter, for example, would be a laundry basket or milk crate filled with wadded up
bird netting. Sit the tub/crate on top of something like a concrete block inside your pond. Put the pump on the bottom of the pond, connect tubing to the output and stick the far end on the top of the tub of netting. Probably a good idea to tie or clamp the tubing end in place so it doesn't flop around and outside the pond. You will want the bird netting wadded up fairly well. Every few days, pull the tube, bring the tub and netting out to the pond - find a good spot in the garden that needs fertilizer and spray it really good with the water hose. Put it back in the pond. Rinse and repeat every few days. After a while, you'll want to start adding a finer netting to start catching the smaller particles, but we leave it there for now!
After you figure out what you've got to work with, and take a look around at some ideas of what kind of ponds are possible, you can start to get an idea of what
YOU want from your pond and what you want to put into your pond.
Oh, and enjoy the journey!
* Laguna and Oase are other good quality low energy pumps but I didn't see them in lower GPH
