how do you make a biodigester that will turn animal waste into energy

 People have been doing this for decades (and maybe centuries).  Search "methane digester" or "biodigester" on Google.

Also checkout youtube.com  
 I should add that the first time you get a load of gas from the digester, vent it off.  This is because it has some oxygen in it from trapped air.  It would explode if you tried to light it.  The subsequent methane produced is oxygen-free (unless you open the digester).