I Want To Invite A Friend From Usa To Spend Holiday With Me In Uk What Do I Need?

Your address
  XYZ street

date:?

Your friends address
XYZ

dear friend's name?
  INVITING FRIEND TO SPEND HOLIDAYS WITH ME IN UK

hi how are you ? I hope fine.And thanks for coming on previous weekends of my school .I am getting bored on these holidays of my school there is lot of fun in UK but I can't do that alone so please come to UK on weekends we both do lots of fun together. There are many excitement places in UK I hope you will come.bye take care
  Your sincerely,
  your name

If your friend has a USA passport their will be no problem.  Send her the invitation letter.  She will have to have a return ticket.