Ok so u cant '+ ' like this in c++ stings
Even I am a novice.But yeah I can help you.
You can subscribe Competitive Programming Playlist in the Competitive Programming Community and if you feel that you need some more knowldege or have confusion for any of the topic, google it out.
Also try to learn STL in C++ .It would be of great use.
You can also checkout with other post on Commonlounge or Quora