First, I recognize the most common place I see this error is in fiction - be it books, TV or movies, so yes, I do understand that to an extent I am yelling at fake people. However, these things are all put together by real people, and based on discussions I have had with real people, it seems that this confusion is not just fictional.
Second, I am neither a lawyer nor official representative of any religious organization. If it turns out your lawyer and/or religious organization are calling these things contrary to me, they may be more right. (Although I would check, they may be more wrong.)
Now, that we have that out of the way...
A guardian is a person who agrees (we will get back to the agree part later) to take responsibility for a child. This can be because the parents (or prior guardians) are ill, incarcerated or otherwise temporarily unavailable, or sometimes because the parent/guardian/prior person looking after the child is deceased.
A godparent is a person who agrees to assist with the child's spiritual growth.
Now, the agree thing - you are supposed to consult with this person to make sure that this is a role they are willing to fill. While you may think your child is the most special snowflake (and, you may be right, I am not disagreeing) that does not mean that all of your friends and siblings have a secret desire to take on this role for your child. And let's face it, you not only want to pick the best person, you want to pick someone who wants to do this.
So, back to the distinction. This means that a godparent should be a person of the same faith as you. (Usually - some faiths work this a little differently). This is not to be mean you your very best friend from high school who you love lots. This is because one imagines that the faith you have chosen to raise your child as is the one you wish to have reinforced.
And the guardian thing, well, that's pretty much up to you, but I think you should pick someone who likes kids. (Actually that's probably a good idea for both.) They don't have to love all kids, but it helps if they like yours. And they should be someone who is aware that you have designated them as such. These are not the kind of surprises you save for will readings. (Okay, sure, sometimes that makes a funny movie, but in real life, no.)