how do u build outposts????

You have to build the second level food structure (the 1st row).

After you build that click on the "Build Outpost" will activate. (the thing that looks like a tent in the upper left.

Then, find an open spot, click, and build.

ok, can you attack places that have already been conquered?

By conquered I'm assuming you mean the colonies but since this thread is about OPs, maybe you meant captured OPs?

You can attack conquered colonies but you won't gain anything by it except killing their units or yours.

You can capture OPs that have been built or captured by other players at any point in time.