i installed the game and Source SDK but cant see game launcher anywhere, there’s no exe file in a folder where i installed it, and it doesn’t appear in Steam… i’ve been waiting this game for so long, and now i wanna kill myself 
If you just downloaded the BlackMesa.7z file for the mod make sure you extract it here.
Steam\steamapps\sourcemods\BMSOnce done restart steam and it should show up.
Edit, also you need Source SDK Base 2007 (Mod Engine), not Source SDK (Mapping tool).
Any luck finding out how to start it?
Edit: Thanks to DaftMink
It uses hl2.exe and its located
Steam\steamapps\your name\source sdk base 2007
Create a shortcut with these settings:
“x:\Program Files (x86)\Steam\Steam.exe” -applaunch 218 -game “x:\Program Files (x86)\Steam\steamapps\sourcemods\BMS”
Where “x” is the drive you installed the programs.
Taken from this thread https://forums.blackmesasource.com/showthread.php?t=13479