Barre chords
Barre chord are a huge stumbling block for probably everyone who has ever picked up the instrument. It is well worth persevering with them though as they are very useful; like almost anything they will improve with practice. They are harder to play near the nut of the guitar so when you first start to practise them, try starting off around the 5th fret or so. Pay close attention to the position of the thumb on the back of the neck. Try to get it directly behind the index finger as this will help you to achieve cleaner sounding chords.
As you become more comfortable with them move gradually closer to the nut. Practise songs that have a good mixture of open and barre chords. Hotel California is a good example. Also take songs that you have always played using open chords and play them strictly with barre chords. For more hints and tips on how to play barre chords read
this and
this. For details on strumming read
this