One 28 cent stamp will do the job if you are mailing your card from within the US.
TWO US Forever stamps will suffice (USA to London post card)
Lots.
4
If this is just a post card you will only need the one stamp to mail it within the US.
7
Postcard has 2 syllables. Post-card
You only need one!
it cost .26 cents
Many sites offer postcard printing. Vistaprint is a very popular printing site with many business card and postcard printing options at a reasonable price.
It only takes a single stamp. But it does have to be the correct denomination. Currently the rate for one ounce from the US to Ireland is 98 cents for a postcard or the first ounce of mail.
usually none, but if it needs a stamp, just one stamp is needed.
No you cannot use US stamps. Items mailed must have the stamps of the country of origin on them. Most shops will be able to provide you with the appropriate stamps to use to mail with their cards, or visit their post office!