Dagesh in Hebrew is simply a dot; but a very meaningful dot…
In English, don’t we have sometimes to pronounce a letter in a certain way and some other times we have to pronounce it in a completely different way? Example? “G” in “god” and “general”. It’s the same in Hebrew…
Back in the day, when Hebrew wasn’t widely spoken, people started to forget how to pronounce certain words and certain letters, so they started using niqqud and dagesh to tell what the letter should be pronounced like.
Anyway, enough chatting, let’s cut it right to the chase… There are two types of Dagesh in Hebrew, Dagesh Kal (Dagesh Lene) and Dagesh  Chazak (Dagesh Forte).

Dagesh Kal – Dagesh Lene

Dagesh Kal (or Qal) is considered a “weak dagesh”, still its effect isn’t weak at all! It can completely change the pronunciation of certain Hebrew letters, 5 letters to be exact:
ב
ג
ד
כ
פ
ת
Here is how these letters are pronounced. I will pronounce them first without a dagesh, then with a dagesh.
בּ    ב
[audio:http://www.israelhebrew.com/wp-content/uploads/hebrew-pronunciation-b-v.mp3]
גּ     ג
[audio:http://www.israelhebrew.com/wp-content/uploads/hebrew-pronunciation-g.mp3]
דּ     ד
[audio:http://www.israelhebrew.com/wp-content/uploads/hebrew-pronunciation-d.mp3]
כּ     כ
[audio:http://www.israelhebrew.com/wp-content/uploads/hebrew-pronunciation-ch-k.mp3]
פּ     פ
[audio:http://www.israelhebrew.com/wp-content/uploads/hebrew-pronunciation-f-p.mp3]
תּ     ת
[audio:http://www.israelhebrew.com/wp-content/uploads/hebrew-pronunciation-t.mp3]
As you notice, the dagesh in modern Hebrew only changes the pronunciation of bet, kaf and pe. The letters gimmel, dalet and tav don’t change the way they sound when they get a dagesh.

Dagesh Chazak – Dagesh Forte

Dagesh Chazaq is exactly what its name says, it’s a “strong” dagesh, in the sense it makes the Hebrew letter sound stronger (it doubles its pronunciation).
But beware! Modern Hebrew ignores that rule… A letter is a letter, it doesn’t get any stronger!
Here are a few examples, I will pronounce the word once with the dagesh sound and another without the dagesh sound.
השּמים (hash-shamayim, hashamayim)
הסּפר (has-sefer, hasefer)
משּם (mish-sham, misham)

How to write Dagesh on your computer

If you want to add a dagesh to a letter, all you have to do is:
1: Press he Caps Lock
2: Press Shift and hold on to it
3: Press the button that shows + and + (usually next to backspace and zero)