"I stand by to abide with (the) sky" to me this makes great sense as part of a traditional/native or sacred legend that the spirit of the dead are at one with the sky (stand by= waiting patiently on this earth to abide with = be a part of eternally).
Seventeen years old = Juushichi (Juunana) sai desu.
I am under the sky = Sora no shita ni imasu. (Literally standing on the ground).
I will become a guardian = Hogosha ni narimasu yo.