i haven't done exactly that, but 3D or 2D is all the same it's just that 2D has one axis locked.
just write a small physics engine and your halfway there, i am sure there are tons of tutorials out there, just google "game physics tutorials"

Sounds like you are nearly there if you have the sprite collisions working! Scrolling maps and platforms are mere extensions to sprite collisions in general.

Honestly, you need the concept (the hard part), not exactly OpenlGL or DX way of doing it (that part is rather easy!)

I'll see if I can give a stab at explaining it a bit. Forgive me if I sway around a bit. It sounds like you already have a lot of the basics, so if I assume too much, please ask questions.

I'm gonna do something basic, using a grid system matrix with 2 dimensions - X and Y.

Let's say the grid is 10 tiles wide by 10 tiles high to keep it simple. Size really doesn't matter.

Start position is center matrix: 5,5

There are walls around the edge of the map:
0,0 to 0,9
0,0 to 9,0
0,9 to 9,9
9,0 to 9,9.

Yeah, a nice plain box!

Each tile is 32px X 32px
Keeping it simple, the Avatar is also 32px x 32px

Let's use the Bounding Box method of collision detection (for those unfamiliar, its a box that surronds the sprite, or tile drawn to check collisions with).

You start at the X,Y location where the avatar is located originally. You game loop is designed something like:

Do

Move Map
Move Avatar - will check collisions against map and various sprites
Move Baddies - will check collisions with map and various sprites
Draw Map
While drawing layers, see which layers to draw baddies and avatar


Loop until exit

The basic idea is you check collisions with the map tiles located on screen against the avatar. To check map tiles such as walls, platforms, etc. use a value for the image. IE: A wall image has a value of 10, water has a value of 5, etc. When you loop through the various tiles, do a quick check of which tile value is assigned, then apply the collision detection. Since you are working on a platformer, use gravity. Make sure you time control it all too, otherwise unexpected results will happen (jerkiness, etc)

I'd suggest that you perform collision checks with avatar and baddies in each baddie move routine. I think that keeps it simpler. You loop through the baddies once vs many times. You have to loop once to check their move, why not check the collisions to the Avatar in that loop? It save s a cycle or two :) Its up to you ultimately, just my two pence.

So basically when the collision occurs with the map and sprite / avatar, you will perform the reaction to that collision. Mostly a stop in movement or a bounce, etc.

Back to the start and wall locations in my example. As you progress across the tiles (we'll say the cross-able tiles are value 0), you'll check the collision if the value is not 0. Once you reach the wall, the value is 10. Then collision will occur and you execute the code that says to make the avatar (or sprite baddie) to stop moving in that direction.

Here's pseudo(ish) code for the Bounding Box:

If (box1_X1 > box2_X2) or (box1_X2 < box2_X1) or (box1_Y1 > box2_Y2) or (box1_Y2 < box2_Y1) then Collision occurs

I am writing a 4-way scroller with multi-layers. It uses the principles I described here. Its a bit more involved than this I described, but this is the basic flow to it all.

You can see it in action here: www.computercoder.org

I hope this helps!

Enjoy your game development!