Travelling to Ukraine?

I am going over to Ukraine (Kiev) in a couple of weeks and am getting rather overwhelmed by guidebooks and websites so would love to have some info from ppeople who have actually travelled there!
My main concerns are the language, how much English is spoken....And is it better to know Russian or Ukranian? Also i've heard the taxi services are not brilliant and not necessarily safe i was hoping to just get into a cab outside the airport - is this realistic/easily done? Do the ukranians welcome foreign visitors or are they hostile at all?
To aid your answer i'm 17, female and visiting Ukraine to do some voluntary work with disadvantaged children :)

I'm English btw :)

First, Just have fun, you will love Kiev!! I lived there for year and it is really the best place!
so Language, In Kiev most people will speak Russian on the streets and some will speak more Ukrainian in the home, if you know one or the other you will be fine. if you going to bring a phrase book I would bring Russian I only used the Ukrainian book with my host family.
Not very many people speak English but some how I was got what I needed. you will be fine, just don't be afraid to ask more than one person for help.

As far as the taxi, I loved taking taxis but, I would not take one from the airport, they will charge way more than they should, there will be buses out side take on of them and ask more than one, you can barter. (you should barter.)

I found that Ukrainians do welcome foreign visitors, I dont know were your from but i am from the states and they loved trying to talk with me . I met a ton of really good friends..

I have been to Ukraine. I speak no russian or ukraine but found most people understood what I wanted (I also had a native with me). Most all speak either Russian or Ukrainian, so either will work okay. Most restaurants have both English and Ukraine menus available. I did not have a problem with taxis but please use those from companies. There are both company and gypsy cabs available at the airport. You may want to arrange for a ride from airport to your destination before you arrive at the airport. Perhaps your hotel or apartment service can arrange this for you. Ukrainians welcome foreigners but there have been racial attacks on those of color recently, so please be careful.

I'm 13 years old . I was born in Ukraine and visit every 2 years . I`m sorry to say that few english is spoken there . Everyone understands russian so you can speak either russian or ukranian. Yes you can get a cab from the airport but I recommend taking a bus or a mini shuttle bus known as a marshutke . I`m pretty sure that the people won`t be hostile to you