I chopped up a pair of size 60 jeans found at the thrift for my first pattern and added about 3 inches on each leg to make them a little bigger. Helped me figure out ideal measurements because I could try the jeans on before sewing to be confident in size. My favorite pair I've sewn was with a 3-layer nylon:
https://www.rockywoods.com/Mid-weight-Nylon-Taslan-3-Layer-Waterproof-Breathable-Fabric. After many days street and park I can say it lasts but definitely hurts the bank. They make a cheaper 2-layer one which is pretty durable too, but it has this rubbery texture on the inside which was sometimes frustrating to sew with (though I'm definitely not proficient with a sewing machine so might have just been a problem for me). A cheap option for a first pair that'll probably last at least to the end of the season is
https://www.fabricwholesaledirect.com/products/polyester-ripstop-pu-coated-fabric and they got lots of colors which is a plus.