Making a transparent background

[Helvetica]

This is probably simple for most of you experienced users, but I can't work out how to do it.
For a new coin catalog that I am compiling, I need to sketch hundreds of monograms with Greek letters. Because some letters are different than those found in normal fonts (even when my keyboard is switched to Greek), e.g. a P (Rho) with a very long | or a very wide Pi or a K with the legs more like =, a flattish Delta etc, I have pre-prepared such letters and stuck them onto a jpg.
However, when I cut and paste them, I get the surrounding white background of that letter so it covers up part of the letter I am pasting it on to.
So I need to make the (white) background of that letter transparent.
Can someone tell me how I can do that ? (just imagine, say, the letter A, on a white background which needs to be transparent).
Thanks for reading..

[hartpaul]

Click the layer and promote image to layer so it reads as Raster and not Background
Magic Wand tool, untick contiguous match mode can be RGB, or Brightness. Click the white areas and hit delete key. You should finish with the black letter on a transparent background.
Save the image as a png to maintain transparency.

[Helvetica]

Super, thankyou Paul. I will try this tomorrow.

[JoeB]

I would add that you might have to also check the Anti Alias box, and maybe a bit of feather, to get a smoother look and/or blend if your first effort doesn't quite do it for you.

[Helvetica]

Thankyou Paul and Joe. I managed to do this, that should save me some time. (It is incredible how many different monograms the old magistrates managed to think up on the coins they issued. I have found over 250 different ones, just on coins of the city of Sardes and they all have to be drawn)