You've already seen that any length of note can be written by tying note-values together. Certain lengths can also be written by dotting notes.
By putting a dot after a note head, you increase the duration of the note by 50%. So, when a half-note/minim lasts for two beats, a dotted half-note/minim will last for 3. The two bars of music below will therefore sound the same. Either notation is correct but the second is generally preferred.