To access the Tower of Sisyphus, you need the hookshot. To acquire the hookshot you need to beat the second boss, Ixion, found in the Crimson Wastes. When beaten, the boss drops the hookshot.
Then return to the start of Overgrown Ruins where Helios is located. In the starting area there is now a ledge you can access by using the hookshot.
Have fun ascending the Tower!