Japanese Slang

I have some question about the japanese slang.
I saw some sentence with the word "し" added in the end of a verb like aru + shi. I also saw a "-ssu" written in katakana, added in the and of some sentence or phrases, and I'd like to know how to use it. メッセージ読んでくれてありがとう！

shi is used in the sentence where giving number of resons

for example
わたしはあたまがわるいし、からだもよわくてだめなにんげんです。
it means iam not very smart and iam weak.i'm good for nothing.

sorry i dont know about ssu

Since this is a question on "slang," I assume you are talking about the young people's language like;
"Ikitaku nai ssu. Tsukareru shi."

It doesn't really mean anything. It's something like;
"I ain't going, dude. Like I get tired, man."

It's hard to explain the usage of a slang-ish expression. You just have to catch the feel of it as you read/hear more of it.

While ramya's explanation covers the PROPER usage (grammatically correct usage) of "...shi," yes, as Uco says, this "...shi" is used quite liberally among young people as a casual ending to their sentences. They don't really fully END the sentence, but they kinda blur the ending and wait for, ya know, people around them to agree, so that they don't have to say anything definite.

"...ssu" is (I haven't really seen it in writing but I've heard it) "...desu" sloppily pronounced, and is also used as relatively informal sounding ending to sentences.

日本語教えてくれてありがとう！
Thank you!

yeah, a lot of young people and kids use 'shi' at the end of their sentences. i work in a junior high school and they always say 'wakaranshi' or 'shiranshi' when I ask them a question. i think it gives it a that young hardass ring to it like 'i dont f-ing know' (and why the hell would I?) but i dont hear that shortend form of desu so much around where I am (Mie) but I hear it when i once talked to a young dude in Nagoya, and when they interview 20 somethings on TV.

Hi! I’m native Japanese.
The word "shi" has "I-have-some-more-to-say-but-I-won’t-do” feeling.
“shi” is just like “and”. So, in using it on the end of the line, we add “non-finished feeling” in the line. In formal Japanese (or maybe in any language), we don't end a line "shi".

Ramya gave us a good sample.
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わたしはあたまがわるいし、からだもよわくてだめなにんげんです。
it means iam not very smart and iam weak.i'm good for nothing.
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If we finish the line in the middle (「わたしはあたまがわるいし。」”I am not very smart and…),we can add many non-finished feelings. But it is very difficult to explain, because it depends on situations.
He may shrink for difficult exit exam and thinks that he can’t even try. =(I’m not very smart so it is no use to challenge this exam.)
Or, he may angry because he made a mistake but don’t want to be accused. =(I made a mistake because I’m fool, BUT SO WHAT!!)
Anyway, using “shi” in the end of the line makes poor impression, and it is bad manner, I think. (Many Japanese teachers or parents hate their children to speak like this.

And, “ssu” has some comical feeling. We don’t use it in serious situations.
It is shorten word of “desu”.
“ii desu / ii ssu”「いいです／いいっす」(By the way, “ii desu (ii ssu)” has both meaning “no thank you” and “it’s OK”. It’s depends on situations…Isn’t it funny?)
“ssu” images me under thirties, low-educated, pout, but intimate person. (It is my own feeling, maybe.)