Answer 1

theres no enemies attached to a goal flag, so theres no way to end the level

Answer 2

Place a red flag (under Entities) then shift-click an enemy

Answer 3

I helpfully suggest reading the help in the editor itself.  It has many tabs of useful information.  Also, try copying another map into the editor folder (levels_src) and looking at it to learn by example / reverse engineer a good map.