If you have entered nothing but ancestors and their children and nobody else (ie peripheral relatives, such as other spouses, other spouses of other spouses, children of other spouses, parents of spouse, etc), you can do a simply ancestor split at:
File > Export > Ancestors
by selecting ancestors of two sets of grandparents, or four sets of great-grandparents.
If you are a normal researcher, you will have all kinds of peripheral relatives that will need to be exported.
To export peripheral relatives of ancestors, you will need to use the ability of FTM to export everyone in a Chart
1) First detach at appropriate points your parents from grandparents or Grandparents from Gr-Grrandparents, etc.
2) Select the mother or father
3) Go to Publish > Charts > Extended Family Chart
4) then right click in blank white space
5) Export all in this chart.
Exporting does nothing to your current file - so you will always have it to go back to.