Need Help, At & T galaxy s6

Hello there, i just purchased a second hand an at & t network unlocked galaxy s6 and want to root and install custom rom like noble rom etc in it because i am tired of At & t bloatware. so i want to know that how i can install it? is there any issues of bootloader locked etc? currently i am on lollipop 5.1.1 and software is up to date.... in odin menu it says

system status: custom
Reactivation lock: Off
knox warranty: 0 (0*0000)

I want a summarized instructions which i couldn't find anywhere else... so posted here.. urgent help is much appreciated

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