I usually select with the magic wand tool, then CTRL U (i think) to bring up hue/saturation, and put the saturation to 0.

if you want for example a little flower on the pic in coulor but not the rest you can select the flower with for examlpe the Magnetic Lasso Tool. Then Select--> Iverse. Then you make the pic black and white. Then Select-->Deselect. done!

That's how I do it........

I always lose when I fight with the magic lasso

How about adding a black&white adjustment layer. Then painting in the layer mask to bring out stuff that should be in colour.
This is somewhat similar to James' suggestion, but here you can also choose how much desaturation you want by painting various gray tones in the mask.

Use the extract tool, in addition of multiple layers. It does the best job of selecting parts that you need.

Oooh great, thanks for all your suggestions, I'll give them a go!

If you have a layer, is there any way that you can turn the layer a bit?
I know how to replace it but not how I can turn it..

Can't you select the whole area of that layer and then just turn it? There is a rotation option in the 'Image' submenu I believe... I don't know if that's what you meant?

Edit: t kan ook zijn da je de rotate optie in de 'Edit' submenu nodig hebt, en dan bij 'transform'

Aha! Found it! Thank you

Yay! I feel so technical right now