I want to start learning how to read sheet music for guitar, so I was wondering if anyone knew of some free software or websites that could help me. When I google it, pretty much just tablature stuff comes up. I can already read trombone, tuba, and bass music well, so i don't need the extreme basics or anything, just which note is which on the staff. Help will be much appreciated.
honestly just find a fretboard diagram attached to a trebale cleff staff.

it helps so much when it comes to reading scale runs.
Quote by Sloopy
I'm not in a wheelchair, but I own one just for fun.
powertab has sheet music as well
Quote by HeretiK538
Totally awesome, I love you.

Have my children.

Buy a book or take lessons. I wouldn't bother with online lessons.
Soon you will sit on the bench
of those who deny I have my soul
You sell a dream you create
Condemned by what you condemned before
Smooth are the words you sing down and high
Underground is your joy your laws
Considering that you already know a bit about reading sheet music, then I would recommend a book instead of software to achieve your goal. A descent publication that a guitarist can guide themselves through is "Hal Leonard - Guitar Method - Book 1":
Guitar Method Book