insert into emp(empname) values('"+textbox1.value+"')


If you use html cocntrol textbox1.Value
or
If you use Aso cocntrol textbox1.Text

insert into emp(empname) values('"+textbox1.Text+"');

Syntax:
       INSERT INTO table name(column name,column name,column
name)values(values,values,values);
Example:
       INSERT INTO
employee(Name,emp_id,designation)values('"+textbox1.text+"',"+textbox2.text+",'"+textbox3.text+"');


..................if u use Html control we must use
textbox1.value ..if u use asp.net controls we have to use
textbox1.text........

in asp.net webform,

first assign namespace like using system.data.sqlclient;

insert coding

sqlcommand cmd = new sqlcommand("insert into address values
('"+ textbox1.text +"', "+ textbox2.text +")", 
connectionstring);

note:

sqlcommand cmd = new sqlcommand("insert into address values
(@textbox1.text,@textbox2.text)",connectionstring);

sqlcommand cmd = new sqlcommand("insert into address values
('"+ textbox1.text +"', "+ textbox2.text +")", 
connectionstring);