How to add libaries such as #include <th1f.h>

Newbie
,
3

after running the command line above it gives me
main023.cc:9:10: fatal error: ‘Pythia8/Pythia.h’ file not found
#include “Pythia8/Pythia.h”
^~~~~~~~~~~~~~~~~~
1 error generated.

where this path should be relative main023.cc