Here is a sample for opening a shapefile. Or did you want to get the workspace from an existing layer in the map?
IWorkspace workspace;
IWorkspaceFactory workspacefactory;
IFeatureWorkspace featureworkspace;
workspacefactory = new ShapefileWorkspaceFactory();
workspace = workspacefactory.openFromFile(filePath, 0);
featureworkspace = (IFeatureWorkspace) workspace;