Pi
To help you remember just draw this diagram.
 
Draw a circle, or use something circular like a plate.
Measure around the edge (the circumference):
I got 82 cm
Measure across the circle (the diameter):
I got 26 cm
82 cm / 26 cm = 3.1538...
That is pretty close to π. Maybe if I measured more accurately?
 
In fact π is approximately equal to:
3.14159265358979323846
The digits go on and on with no pattern. π has been  calculated  to over two quadrillion decimal places and still there is no pattern to the digits
Example: You walk around a circle which has a diameter of 100m, how far have you walked?
Distance walked = Circumference = π × 100m = 314.159...m
= 314m (to the nearest m)
Approximation
A quick and easy approximation for π is 22/7
22/7 = 3.1428571...
But as you can see, 22/7 is not exactly right. In fact π is not equal to the ratio of any two numbers, which makes it an irrational number .
A better approximation (but still not exact) is:
355/113 = 3.1415929...
(think "113355", then divide the "355" by the "113")
Remembering
I usually just  remember "3.14159", but you can also  count the letters of:
"May I have a large container of butter today"
3 1 4 1 5 9 2 6 5
To 100 Decimal Places
Here is π with the first 100 decimal places:
3.14159265358979323846264338327950288