Anyway, dark lyrics are overused, somebody needs to be more upbeat.
Exactly my thoughts, hence i'm in the process of forming a band that does just that! ahaha
Just as if you were playing an instrument, my opinion of writing lyrics and songwriting in general is that you have to practice - you will start off shit, but only through practice will you become better.
To get you started you could try copying your influences completely. I advise not to stick to this, but merely as a stepping stone of sorts. At first you'll sound like a total clone, and most likely a horrific clone, but over time you should develop your style - learning what you're good at conveying and a way you enjoy writing in. It will most likely take a while, and during the development your lyrics may suck big time. Try and copy as many different lyricists as possible so that you may be able to glean a little something from each of them! It's better to sound like a combination of Akerfeldt/Wilson/Waters (for example) than just a carbon copy of one of them.
Reading books and poetry will also give you a greater appreciation of description, putting across your message and structure etc... I wish i had done more of this, and it is something i hope to do more of. But yes, the more you read, the more accustomed you'll be to ways of writing and it should be reflected in your writing. It will hopefully expand your imagination..
..As well as watching films and going out on walks or trips. Not just dross films, but artistic films with good scripts, complex plots, interesting directing and camera-work etc.. And not just a stroll down your shops - go to your local park etc.. Go to the beach.. Whatever - just try and experience things for yourself to give you ideas of things to write about! Mereley sitting around on your computer doesn't exactly lend itself to writing much about forests and what not. If you're too lazy (not suggesting you are) just looking up pictures of such things may be the best alternative available.
A good idea would be to take either a pencil/some paint and a pad of paper and do a drawing/painting of what you see, or even just write a description of it.
Okay, i'm spent for now.