I made a shoot em up like Gradius a while ago, lots of powerups and weapons, good fun to make.
I think the techniques you use depend really on how complex the levels will be. If you are good at art, then I suggest making big blocks and layering them. So you might have a beam sticking out of the ground, but then you would have other blocks to go on top of that, for turrets or other graphics. If you don't feel confident enough to work like that, then making tiles is the best option. You might start with a repeating tile, there's lots supplied with DBPro so best to have a look at those. You can make lots of copies and change them to create different structures. You might have a steel block then make different versions for the sides and top, so you can build these blocks onto a grid to create the level layout.
It's probably a good idea to look for some sprite sheets from games like Gradius, might give you an idea of how to lay out the graphics. It might be hard to find backdrop sprites, but there would be plenty showing enemies and stuff.
Personally I wouldn't use pure 2D for a project like this, I'd lay out the level as plains and have a parallax effect, but I'd use a grid too for some parts. There's also sprites of course which offer some benefits over normal 2D image pasting, like alpha fading and rotation. Collision is always a problem, so a pixel perfect sprite collision example would be a good starting point I think. It can be complex to make your own collision system. In my shoot em up I used line to line and circle to line collision. This way I'd draw this line in segments around the block, and enemies would have these outlines too - then bullets would be checked as a line between the previous and current position. Pretty tricky to get all the work done to edit the lines etc, but worth it as it's a pretty fast and useful technique.
