How can i create a new dropdown list

Hi, well on project i want to create a new field, that is a kind of select, dropdown with some values from the database. I have created a new table on database (table Country for example) and then a new field on the table Project idCountry. Then on the process of creating a new project i want that the field idCountry displays all the values from table Country. Which are the files that I have to change or see? How is this connection done on projeqtor php code?

I declared the variable idCounty on ProjectMain and it looks like this.. why the other elements disappear?

Yes, but I almost did it, can you help me?