Automating Incrementation In Primary Key ID Field

I'm really puzzled with this. I am new to using MS SQL and need help with the primary key. When I was using Access, I would open the database table and just enter values for the various columns, each time a new column was begun, the computer would insert a unique number there. OK, so now I am using visual studio to work on a SQL database. I created a new table and the first field I specified was "ID", I then chose it as the primary key. For default value, I tried both auto number and auto increment. These don't seem to work, as when I type values into fields and try to move to the next row, having left the ID field blank, I get an error which reads "String or Binary data may be truncated". What is going on, what do I need to do so the field automatically populates with an ID number... Some research sort of led me to the concept of a "trigger" to do this. Is this the approach. I was able to r-click on the table and I saw an option for "create trigger" or "new trigger"... How do I learn to do this, is there a generic trigger to do an auto increment of a primary key ID field?