If you want to do a mass-mailing, use mail merge:

See the article "How to create a Mail Merge" at:

http://word.mvps.org/FAQs/MailMerge/CreateAMailMerge.htm

and Creating a Mail Merge Data Source" at:

http://word.mvps.org/FAQs/MailMerge/CreateADataSource.htm

If you want to create just one letter at a time, the best thing would be to
create a template with a userform in it for your letter

See the article "How to create a Userform" at:

http://word.mvps.org/FAQs/Userforms/CreateAUserForm.htm

Then if you copy and paste the addresses into a Word document, or use a
catalog or directly type mailmerge to get them into a Word document, you can
use the following information to populate a listbox or combobox on the
userform so that you can select individual addresses

This routine loads a listbox with client details stored in a table in a
separate
document (which makes it easy to maintain with additions, deletions etc.),
that document being saved as Clients.Doc for the following code.

On the UserForm, have a list box (ListBox1) and a Command Button
(CommandButton1) and use the following code in the UserForm_Initialize() and
the CommandButton1_Click() routines

Private Sub UserForm_Initialize()
   Dim sourcedoc As Document, i As Integer, j As Integer, myitem As Range,
m As Long, n As Long
   ' Modify the path in the following line so that it matches where you
saved Clients.doc
   Application.ScreenUpdating = False
   ' Open the file containing the client details
   Set sourcedoc = Documents.Open(FileName:="e:\worddocs\Clients.doc")
   ' Get the number or clients = number of rows in the table of client
details less one
   i = sourcedoc.Tables(1).Rows.Count - 1
   ' Get the number of columns in the table of client details
   j = sourcedoc.Tables(1).Columns.Count
   ' Set the number of columns in the Listbox to match
   ' the number of columns in the table of client details
   ListBox1.ColumnCount = j
   ' Define an array to be loaded with the client data
   Dim MyArray() As Variant
   'Load client data into MyArray
   ReDim MyArray(i, j)
   For n = 0 To j - 1
       For m = 0 To i - 1
           Set myitem = sourcedoc.Tables(1).Cell(m + 2, n + 1).Range
           myitem.End = myitem.End - 1
           MyArray(m, n) = myitem.Text
       Next m
   Next n
  ' Load data into ListBox1
   ListBox1.List() = MyArray
   ' Close the file containing the client details
   sourcedoc.Close SaveChanges:=wdDoNotSaveChanges
End Sub

Private Sub CommandButton1_Click()
Dim i As Integer, Addressee As String
Addressee = ""
For i = 1 To ListBox1.ColumnCount
   ListBox1.BoundColumn = i
   Addressee = Addressee & ListBox1.Value & vbCr
Next i
ActiveDocument.Bookmarks("Addressee").Range.InsertAfter Addressee
UserForm2.Hide
End Sub

The Initialize statement will populate the listbox with the data from the
table and then when a client is selected in from the list and the command
button is clicked, the information for that client will be inserted into a
bookmark in the document.  You may want to vary the manner in which it is
inserted to suit our exact requirements, but hopefully this will get you
started.

To make it easy for you, the code has been written so that it will deal with
any number of clients and any number of details about each client.  It
assumes that the first row of the table containing the client details is a
header row.