>

Yeah, it's weird that they require a middle man telling them to be good or they'll be punished.
You can just be good for goods sake.
Why only these two choices? Sikhism also has a great tradition of charity and humanism. >

Sikhism isn't too big from where I am located. I thought about Buddhism for a bit, but I ultimately decided on becoming a secular humanist. >

Fair, but why is becoming a Christian in the mix? It sounds like you've made a decision.
I would argue that you're still free to take inspiration from Christianity, but then it sounds like you're interested in the world (hence your research into Buddhism and Sikhism), so why is it necessary to pick a religion? I'm sure Christians would welcome you either way if your intention is to help people, no? >

It is, but Christians based their morality on a higher being. I been to bible study sessions before, and I had to interpret archaic messages and scriptures, to which I found to be a very difficult endeavor. 
It might just be a "me" problem since I was never adept at literary analysis. 
The secular humanist's moral framework seems to be very straightforward: do good acts for self-fulfillment, not to appease a higher power. >

A question I would ask is who is defining "good acts"? Christianity has a basis for moral objectivity. Regardless, you are free to choose. >

You can do everything a Christian does without being a Christian if you don’t want to.
Being a good person isn’t exclusive to being religious. Heck many religious people are very questionable.
The difference is like you said, a belief in a higher power or a belief in humanity and striving to be a good, kind person. >

That's a good point. I just don't understand why so many people chose to become Christians when they can be a secular humanist. >

Uh what? Because they believe in Christ, my dude. The very definition. You seem to be more hung up on labels than most religious people I know. Be atheist if you want and do good. It’s as simple as that. 
They’re not becoming Christians JUST to help the poor. The many umbrella religions come with so much extra baggage. I’m not Christian myself, doesn’t mean you need to be so ignorant (and judgemental).