For newbie or beginners players , here some of the advice I can give .

1.) Start of with 3 hero that you wish to master and practice it often .
2.) Must know the hero in terms of skills and role that you choose . ( You cant expect a support hero like wisp to kill , do you ? )
3.) Once you understand your roles , play it well . Eg. If you are a tanker , always go front . If you a supporter , always stick to your carry . Etc ...
4.) Have some map awareness and constant report . ( Even sometimes mistake report is ok but dont over do it as it will make your team have more alert )
5.) Do not feed .
6.) Help out in most of the ganks .
7.) Do not hesitate too long especially when you carry .
8.) Never purposely ks other people target especially carry target unless they cant make it .

Of course there is more I can give but you will learn more when you got experience .