Photoshop Help!!!

Alright I'm using photoshop 10.0 at my school to make some wallpapers and cards. All I really want to know is how to I make a layer transparent? I basically want to make a photo with a background photo seeping through it making it transparent. I have read some tips on theO and online but nothing is helping understand and I have tried sooo many things
Heres an example of what I mean:

As you can see (look really hard) there are some transparent effects throughout the photo.
Please help me understand layers better!

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