step1:
lets start out by finding an image that has some buildings (or whatever it is that you want to write on). below is the image that i selected to use (it is from stock pack 005 on this site):
step2:
select your text tool (or hit 't') and type what you want to write on the building. be sure to select a good font and a nice size that will look good with the image. in this example i am using the font horrendous @ 18pt:
step 3:
now that you got you image looking a little something like the image above we need to make sure that is is somewhat level with the ground. with the text layer selected goto edit > transform > rotate and move it around until you see fit.
step 4:
you may like the way it currently looks but you can do a little bit more to make it look better. with your text layer selected goto layer > rasterize > layer. then in the layers palette and set the mode of the layer to overlay. (if you want you can change the opacity a little bit too). i moved my text behind the tree a little.