Tell me who your friends are, and I’ll tell you who you are.

"Tell me who your friends are, and I’ll tell you who you are."

this means that whoever we are going with and staying with everyday, their attitudes, characteristics and actions can also become ours too. For example, if we are with good and righteous people, then we are also good and righteous people because we will/can acquire who they are and what they are. With this, we should learn how to choose carefully the people we will go with.