The Enotes page on iambic pentameter (to which I have provided a link below) does a great job of providing simple examples of scansion of iambic pentameter.
The first thing to understand about iambic pentameter is that is has a distinct rhythm and a particular length that is prescribed for each line. The rhythm can be written as a line like this:
da-DUM, da-DUM, da-DUM, da-DUM, da-DUM.
This line shows not only the rhythm (with the bolded "DUM"s being the stressed sound), but also the length of a standard line of iambic pentameter verse -- five feet (each "da-DUM" being one foot), and each foot containing two syllables ("da" and "DUM"), one stressed and one unstressed.
This rhythm (da-DUM) isn't a random choice, either. It suits the natural rhythm of the English language and is very similar to the rhythm of our heartbeat. So, it is written in a way that makes it easy to speak out loud. This is one reason it was used by Shakespeare for his plays.
Here are the opening lines from the General Prologue:
WHEN APRIL with his showers sweet with fruit
The drought of March has pierced unto the root
And bathed each vein with liquor that has power
To generate therein and sire the flower
Here's how to begin your scansion. Please consult the Enotes iambic pentameter page for the correct symbols to indicate stressed and unstressed syllables. I will, for this exercise, place the stressed syllable in bold and leave the unstressed alone.
When A-/ pril with/ his show-/ ers sweet/ with fruit
The drought/ of March/ has pierced/ un-to/ the root
And bathed/ each vein/ with li-/ quor that/ has pow'r
To gen-/er-ate/ there-in/and sire/ the flow'r
I have used the backslash to show the division between feet, and each foot has two syllables. This sometimes divides one word into two separate feet, so I have indicated the division between the syllables of one word with a dash. Also, the "power" and "flower" at the end of lines 3 and 4 really both have an extra syllable, "-er" at the end (called a feminine ending), but I have made them one syllable by taking out the "e" in each one, so they read as one syllable rather than two.
Hope this helps! It's really pretty easy, once you get all the symbols together to indicate your feet, and stressed and unstressed syllables.