I agree with the above poster ^
Give it a good bath with shampoo and conditioner, brush it, and braid+bag or at least just braiding it really helps..
For a detangler I very very highly recommend Absorbine Santa Fe. I've tried Show Sheen, Cowboy Magic, Survivor, etc etc, and none of them held a candle to Santa Fe.
The prices vary..for the spray-on I think around here we get it for about $15..? But if you're keeping the tail braided you won't need it much, so it'll last you.
If you still want to cut it, just be very careful not to cut it too short because you'll REALLY regret it, and it'll take a long time to grow back. Keep in mind that she will carry her tail higher when she's moving.
To cut it, if you happen to have clippers that's the best way to get through a thick tail. If not, just slowly work through it with the heaviest normal scissors you have, and just take your time so that you can be sure to even it out.
Place your hand loosely around the entire tail at the very base, then slide your hand straight down. Get a tight grip around the tail right above the area that you want to cut, then bring it up to eye-level and cut as straight as possible. Sometimes I wind up touching it up a bit after the initial cut, and it's likely you'll have to do that too with such a thick tail.
Good luck =) she's a very pretty girl!