Dark Physics is a good starting place,
you could also write your own simple physics for falling etc.
the first thing you need is collision boxes, if you have them working for moving in the x direction (horizontal) then getting the same collision tests to work y (vertically) is easy.
a nice simple way of doing collision is using dummy invisible cubes as your collision items, create one as your actor and then test that against the other invisible boxes.
there are lots of examples of similar techniques around.
create a cube,
start of game loop
store the cubes last location
move the cube using the keys or whatever controls you use,
test the cube for collision
if its hit something reset the stored cubes location
store the cubes last position
test under the cube by moving the cube down a little
if it hits something your stood on the ground so restore the cubes position
if it didn't hit anything then your falling, you can put simple acceleration to the cubes fall rate etc here, to be added to the next test.
etc.
etc.
move the camera to the cube
go back to the game loop
I'll not go into too much detail, this is just to give you an idea
