Walnut Creek is the best spot for beginners. It's not very technical and heavily travelled. That means you'll have a better chance of being able to pick up other riders and have people show you around. WC is a tangle of trails that go all over the park - they are two way trails, so keep an eye on where you are going.
There are 3-4 creek crossings that can be anywhere from bone dry to a foot deep depending on the time of year. Watch out for roots and loose dirt. Because it is heavily travelled, WC can be subject to heavier erosion so stay on the trails, don't widen ruts and do NOT ride it after it has rained (always ask first).
Much of the trail work in the past few years has focused on helping remove some of the drainage issues, in addition to adding more berms and smaller features. WC drains better now (2020) than it has in the past, but as always, give it some time after a rain.
Location: Located Southwest of the intersection of Parmer and Lamar in North Austin.
Directions: From I-35 exit Parmer and head west. The first stoplight is Lamar, go south and turn in on the right just past the Vietnamese temple at Yager. Park is clearly marked. From MoPac exit Parmer and headc east. About 1/4 mile before Lamar, take a right on Willow Wild and follow that into the park.
Technical Level: Low, perfect for beginners. Few ledges, not too many climbs, no problem for hardtails or light suspension. Lots of trees, some narrow passes and tight turns so watch your handlebars.