what is the main advantages of buying the masternodes and ... what can we get if we hold them long !!
The premise is that you can earn passive income by holding/running masternodes (MN). Why? Because a part of the mining reward for each new mined block is shared between the MN holders. However, you may also have recurring expenses (running the node 24/7, etc.).
how to choose a good masternode , how can we know that '' good masternode is this ''...
This is exactly the point. It doesn't always work, and certainly not will any random masternode coin. Example: the coin may go down in value, so you lose money even though you may own more of the coins.
how can we findout that the project is good or scam 
A good place to start is https://masternodes.online/ Hint: choose well establish coins with decent price history.
Choose a good VPS provider as well. In my experience, it's much better to use a VPS than setting up your own PC at home as MN.
Vultr, Digical Ocean, Node supply (Full service VPS) can be selected.