Creating tables in SQL Server

Alter table table1 add constraint <name of constraint>
FOREIGN KEY (<column>) references table2 (<primary key column>)

Foreign constraint prevents invalid data from being entered into the foreign key column. The foreign key value must be one of the values contained in the table it references.

Creating a table using sql

Use [Sample]
Create Table tblGender
{
ID int NOT NULL Primary Key,
Gender nvarchar(50) NOT NULL,
}

Check the gui way of adding a foreign key constraint

Difference between nvarchar and varchar :
varchar: Variable-length, non-Unicode character data. The database collation determines which code page the data is stored using.
nvarchar: Variable-length Unicode character data. Dependent on the database collation for comparisons.