One of the many dragons that you may be called to slay as a bounty posting by a Jarl- the Jarl of Falkreath, in this case- is the one located at Ancient's Ascent.  This dragon is most easily reached from Helgen, of the various locations that are usually marked on your map.  Pinewatch, which you may have located if you've wandered around the area, can provide you with a closer point, though, and is where you should set out for from Helgen.  This cabin is located midway between Helgen and Falkreath, practically on a direct line between the two.

If you're a sneakier type, you definitely want to start from Pinewatch.  To reach Ancient's Ascent from behind, head due South until the climb becomes Follower-untenable, then divert to the right until your companion(s) can rejoin you.  Continue Southwest, stepping to the right of the fallen log, and take the switchback marked by the pile of rocks.  You should quickly reach a second pile of rocks with a cloth fluttering from it, and after passing a second you'll come to a hunter's camp.  This marks another switchback, so poke around if you like, then climb the mountain heading away from the camp.

Keep going past the bear traps and flagged rocks, taking the next switchback where the snowberry bushes grow.  This will bring you to an area actually somewhat Southwest of Ancient's Ascent, which is where you really want to be.  From here you can head straight for the dragon's location, coming out uphill from and behind the creature's perch at Ancient's Ascent.

To approach from a downhill position, head East from Pinewatch.  You'll find a cobble path leading South past a bandit camp- follow this until you reach a spot where two paths on the right leave the cobble road and join each other heading up a small bluff.  This brings you to a large level area, at the far end of which is a path up the mountain under an ancient Nord arch.  By the arch you'll find a vein of silver ore, and behind it is Bonechill Passage, the main route for reaching Ancient's Ascent.