I am a beginner, I installed oracle 11g R2. I also created a database.
Unfortunately, I am not able to create a connection to databse in SQL Developer.
Sorry - unfortunately you forgot to post ANY info about exactly what you are doing, how you are doing it or what info you are entering in the connection dialog.
Until you do you won't get much help.
I have also faced this issue several times, following came to rescue 99% times (rest 1% solutions I don't remember as of now )
1) Please check the connection info you are giving. Try easy connection too.
2) Please make sure the DB is up and running at server end.
Are you using SQL Developer in the same machine where you have installed the database or trying to access it from outside