In 6 months you can learn a great deal...or you can learn nothing (trust me...I know...I tried to learn Spanish in Canada before I moved to Spain and I learnt...nothing).
I only learnt Spanish when I was surrouned by it. So I think one key thing is to surround yourself with English.
I would suggest (for listening)
Watching tv series (DVDs) with subtitles in English. At first it will be hard and frustrating but TRUST ME it will get easier.
Watch movies with subtitles
Listen to the radio in English all the time (in the car, on the bus, while you work or study) Just have the radio on. Even if you don't listen to it conciously, your ear will get more comfortable hearing English and you will be able to understand more.
I hope that helps.
Have a great day!