This song has a lot of black American English to it and she speaks fast, so it may sometimes be really difficult to understand (even reading it), but one thing I guarantee, by the end of the song you will be quite fluent in the sentence "I whip my hair back and forth" :)

Note: remember that black AmE doesn't pronounce the R if it's not followed by a vowel (the same as BrE). This song is not pure black English, so sometimes she follows this rule and sometimes she doesn't.

WHIP= To use a whip moving it forcefully "back and forth". A whip is a strap of material usually tied to a little wooden pole used to make animals move or work faster (see picture).
If you whip your hair, you move your hair very vigorously "back and forth" to make your hair move around fast, as if it was a whip.

MY= Notice that most of the times this sounds like MA' /mɑ/instead of MY /aɪ/. This is very common in black AmE, but also in colloquial English in general (the same with the pronoun "I").

BACK AND FORTH= (also "backwards and forward" or "to and fro") A phrase to describe a movement that goes to the back and then to the front again and again, like a pendulum. For example if you sit on a swing (see picture of a swing) you move back and forth. A ship also moves back and forth in a tempest.

REAL GOOD= In this case it means: very strongly, really forcefully. But the phrase "real good" has a wider meaning and it simply means "very well".

HOP= Jump.
Hop up = get up (from bed) with a jump.

SWAG= (coll. AmE, also SWAGGER) Your swag is your presence, physically and mentally, so it comprises your looks, your make-up, your clothes… and also your self-confidence. You swag is always a cool positive thing, if you are dressed terribly bad and you feel depressed, you don't have a swag, you have a problem.
To "turn your swag on" means to activate all your best mental and physical resources to be cool, stylish and feel on top of the world (here "turn on" is the same as in "to turn the light on", to activate).

THEM= (very coll. or black AmE) The (plural), those.
- Look at them = pronoun
- (coll.) Give me them books = article

'ATERS= (black AmE) HATERS (black American English often drop the H). The word HATERS is used in black AmE to mean stupid people who love ruining your day. So if a girl goes to a bar to have fun and some guys come to try to flirt with her and she doesn't want to talk to them, she would think they are "haters" (not because they hate, but because they are a nuisance and she doesn't like them)

'EM= (coll.) Them.

WE WHIP 'EM OFF= To whip them off is to shake your hand (or a real whip) to make an animal go away, for example a fly. The particle OFF expresses separation, not distance, so if you whip an animal off, it must be an animal that is in contact with you, for example a fly when it is sitting on your face, but if the fly is flying, then you whip it away, not off (or at least in correct English). Here she uses OFF talking about the boys because they are so close to her that she feels they are in contact with her body (or maybe they really are).

AIN'T= (coll.) The negative form of the present of the verb "to be" or the verb "to have":
- I ain't got no money = I haven't got any money
- You ain't seen her = You haven't seen her
- He ain't no bloody doctor = He is not a doctor
- I ain't English = I'm not English

NUTTIN' = (black AmE) Nothing.
Pronouncing T instead of TH is another common feature of black AmE. In this case, she pronounces the T back at her throat, not with the tip of her tongue. But that is a very common way of pronouncing the T both in the UK and in America, and it's getting more and more popular.

DON'T TELL ME NOTHING= (coll.) Don't tell me anything / Tell me nothing.
Double negatives are considered wrong in standard English, but they happen a lot in colloquial English, and it is the correct way to put a sentence into the negative in black American English.

TRYNA= Trying to.
This spelling is colloquial AmE, but the pronunciation is quite normal in many parts of the States, since there is a strong tendency to drop the T when it goes after an N (N+T= N) so:
TRYING TO = TRYIN' TO = TRYNA
(both the O and the A here represent the weak sound /ə/)

WHAT'S UP= (coll. esp. AmE) Hello.

TO SHAKE 'EM OFF= To make them leave. (same as "whip them off" before)
Guys are trying to pick the girls up (to flirt with them), so they whip their hairs to send them away much in the same way as you shake your hand when you want to make a fly who is on you go away.
(have you ever tried that, girls? Does it work? :D

OFF MY GRIND= If someone keeps/gets/takes you off your grind, they drive you crazy (they make you feel crazy). This can be something negative (like here) or positive:
- Shut up, you're getting me off my grind! (negative)
- Ooh, that girl is fantastic. She's really taking me off my grind (positive)

(I) KEEP MY HEAD UP= If you keep your head up, you feel proud and you feel or pretend to feel superior (or at least not inferior) to those around you.
She pronounces "head up" very quickly, that's why it sounds almost like "hea' 'p".

FINE= Black AmE has a special way to pronounce F, stronger than standard English, so in some positions (like here) it sounds some way between F and P.

(I) KEEP FIGHTING UNTIL I GET THERE= I continue trying hard until I get what I want (until I reach my goal).

I'M DOWN= I feel sad or depressed.

GIVIN' UP= Surrendering.

I'MA= (coll. AmE) I'm going to.

GET MORE SHINE= If you are good and get more shine, you get better, more important and or stylish.

IN A LITTLE BIT= In a few minutes, very soon.

SOON AS= (coll.) As soon as, when.

'IT= (black AmE) HIT = (coll.) Go to.
- let's hit the floor = Come with me to the dance floor, let's dance.
- hit the road, Jack, and don't you come back no more, no more, no more, no more = Go away Jack, go back to your car and leave me.
- Can I hit your house tomorrow? = Can I make you a visit?

THE STAGE= A raised platform where singers sing in front of the public (or actors perform, etc)

WHETHER= If.
In standard English, this word is used with OR NOT, but here the OR NOT is not present, it's implied, so the correct sentence should be: WHEATHER it's black stars OR NOT, OR WEATHER IT'S black cars OR NOT, I'm feeling it".
To know if we can use WHETHER instead of IF, see if you can add "OR NOT" at the end.
- I'll do it whether you like it or not. = I'll do it if you like it or not
- Tell me whether you need it (or not) = Tell me if you need it
- I can tell you if you come (here we can't add "or not" without changing the meaning)

BUT CAN'T NONE OF THEM…= But none of them can…

I GETS IT= (nonstandard) I get it.
Adding an –S where you shouldn't or not adding the 3rd person singular –S when you should is common in black AmE and very colloquial AmE.

I GO HARD= I am very self-confident and press to get what I want.

PULL UP= Stop.

REAL HARD= (coll.) Really hard, with a very big strength.

IF YOU FEEL ME= (coll.) If you agree with me, if you like what I'm saying, if what I'm saying makes you think that you feel the same about this.

GO AND DO IT= Do it.

DON'T MATER IF…= (coll.) It doesn't matter if…

If you want to learn more about Black American English you can go here: Black American English.