Tied notes are written with a curved line connecting two notes that are on the same line or the same space in the staff. Notes of any length may be tied together, and more than two notes may be tied together.

Here is an example showing how notes are joined together by ties.

A semibreve joined with a crotchet

When notes are joined by ties the beats of the notes are added creating one sound. In other words the sound is as long as the number of beats for each note when they are added together.
The tie goes from the head of the first note to the head of the next, on the other side.