How to join only one column?

Assuming that you mean "select one column from table 2":

   SELECT table1.*, table2.first_name
     FROM table1
LEFT JOIN table2
...

The accepted answer is the correct answer but I have encountered a strange error when the tables are in two different databases:

Assuming that table1 is in database1 and table2 is in database2. Initially I have tried this:

SELECT *, database2.table2.first_name 
FROM table1
LEFT JOIN database2.table2
ON database1.table1.id = database2.table2.table1_id
WHERE table1.id = 1

The strange thing is that if I try this query from PHP PDO there were no errors but the result contained all columns from database2.table2 (expected only first_name column).

But if I have tried the same query from phpmyadmin got a sintax error:

Table 'database2.table1' doesn't exist

So, for solve that, then all databases need to be specified implicitly like this:

SELECT database1.table1.*, database2.table2.first_name 
FROM database1.table1
LEFT JOIN database2.table2
ON database1.table1.id = database2.table2.table1_id
WHERE database1.table1.id = 1

Take your original code and substitute * with table1.*, table2.YourChosenColumn

 SELECT table1.*, table2.YourChosenColumn
 FROM table1 LEFT JOIN table2 
 ON table1.id = table2.table1_id 
 WHERE table1.id = 1