what is the meaning of these signs in the middle of a sentence :(? :-(? & where can i get those...to learn?

Answer 1

These are smileys, they represent the human face, eyes nose and mouth, and are used to communicate mood or emotion.

You can make your own with some imagination :

:-) = happy
:-( = sad
:-/ = confused
;-) = blink
B-) = sunglasses

Some variant include eyebrows

>:-) = evil smile
||-( = very perplex

Some are just drawings :

|__|______//// = cigarette

( . Y . ) = breasts

Answer 2

These days they are more often called "emoticons", because these little keystroke picture, most sideways depictions of human facial expressions (etc), are mostly used to express moods and emotions.

Here are a couple of good lists to check out:
http://www.computeruser.com/resources/dictionary/emoticons.html
http://www.sharpened.net/glossary/emoticons.php