The first vertical stage, the cat bell can make this climb a breeze!
If you want a preview of the level, you can climb the nearby trees to find some sprixies with binoculars (and the first green star.) Otherwise, head forward and start climbing. The ? block above the first piranha plant contains a bell, and a hidden ? block behind the second carries a 1-up.
Up a little ways from there is a pow block in the wall hiding a green pipe. Hit the switch and collect all the coins to get the second star. Once you leave, the stamp is in plain sight, so grab that as well.
The third cat goomba up from the stamp holds a spare bell. Grab it if you need it, otherwise keep climbing. Near the summit, climb the tree and spin the cat wheel to raise a tower leading to a hidden area. Before you take the cloud to the third green star, jump right and scurry up the wall for a 1-up.
Back on the main path, you'll need to defeat a giant piranha plant to reach the flag. Either wait for it to lunge or tap it to stun it, then defeat it with a jump or other attack. At the goalpost area, you'll need to be a cat to reach the top of the flag. Fortunately, there's a cat goomba holding a bell nearby (and the third tree holds a mushroom.)