Pre-scored card blanks or A4 card to fold yourself.

You can buy a variety of sizes, colours, various edges and apertures.  I recommend A5 plain white cards to get you started.

A good pair of scissors.

A good pair of scissors is essential!  I now have pairs of different sized scissors, but one pair is enough to get you started.

Double-sided sticky tape.

This really is an essential.  Wet glues will make backing papers wrinkle and take time to dry.

Sticky foam pads.

These will give your card a 3d effect.  They can be used just on one part of the card, or on all of it.  Having a handmade greeting card with a 3d element makes it look more professional.

Backing papers.

You can buy backing papers or find them free on the internet.  There are probably millions of designs!  Just choose one that you like.

Toppers and embellishmets.

Toppers and embellishments includes anything from a large picture to stick on your card to small gems and sequins.  Make sure everything coordinates.

Sentiments.

This is the part of the card that says 'Happy Birthday', 'Happy Easter', "Golden Wedding Anniversary", etc.  You can buy sheets with sentiments that you just peel off and stick on your card, or you can make your own using your word processor.

A decent printer.

If you are going to print backing papers from the internet, from CDs, or from papers you design yourself then you need a decent printer.  It soesn't have to be top of the range, and if you already have a colour printer then it is probably good enough for your card making needs.