[Extremely Confused] Rooting the T-Mobile Galaxy S5 and flashing Cyanogenmod 12.1

Hi I recently got a T-Mobile Galaxy S5 and I despise Touchwiz. It looks ugly and more importantly it makes my phone extremely sluggish (it feels like my Moto G when it would run out of RAM). I want to root and put a ROM that's as near stock as possible (I'm assuming CM12.1 is the best option for that) but I haven't rooted since the Galaxy S2 so all the information I'm reading is super overwhelming. All these stickied posts seem rather outdated coming from like 2014 and such. I'm on 5.1.1 and I honestly just want as lagfree of an experience as possible. Can someone here please help me acheive this? I've been researching for 2 weeks now and I'm horribly confused. I keep reading about custom kernels and all that but all I want is my phone to not suck.