1) If you're going to flash a ROM, don't waste time rooting. You root the ROM - when you flash a new ROM, what you did to the old one no longer counts, and almost any ROM you flash will already be rooted.
2) The way to choose a ROM is pretty simple, but it's not something someone else can do for you.
What ROM has the features that you want that aren't in the stock ROM, while retaining all the features from the stock ROM you want. (If you never use a nav app, you don't care if the new ROM doesn't support GPS.) Also is the new ROM stable, does it have bugs, does it crash, does it run with all the apps you want to run?
Most people who just "pick a ROM" for one or two features are dissatisfied with it eventually.
And many of the things that a newbie thinks he needs a ROM for, he doesn't. You can change the whole look and feel of the phone by putting in a new launcher, new dialer, new contact app, new lock screen, some wallpaper and some widgets. You can get a lot of them free at the Play store. Although we have a wallpaper section here that I've gotten some of my best wallpapers from (I try to always have one for the current or upcoming holiday - I have an Androchaun [Android Leprechaun] holding a pot of gold at the end of a rainbow for St. Patrick's Day. Haven't found one of an Android picking someone's pocket for April yet, though. Or any Android Bunnies for Easter.)
But you don't need a ROM to trick your phone out in finery. I use the Nova Launcher and almost everything on my main homepage is in folders. I'm running the Kit Kat Message and email apps. My phone's mother wouldn't recognize it. Stock ROM. I don't need anything not in that one.
A side note. Even rooting a phone can brick it (though some one-click roots are pretty well known to be good). Flashing a ROM makes it even easier to brick the phone. And either one, rooting or flashing a ROM, voids the warranty. Playing with a $50 phone is one thing (and there are still Gingerbread phones for that or less), playing with a $500 phone is another. (And if you ruin the phone you pay full retail price for the replacement, not the subsidized price you paid when you got it. Even the "free" phones are about $100.)
