I've put numbers next to the corrections so you can see the explanations for why they were wrong and why they're what they are below your paragraph by matching them to the number.

First notebook entry: Introducing myself

hi,
I always feel that the worst thing in the world is introducing ourselves (1). I believe that (2) you (3) have to be known by everyone (4) without any introduction about ourselves (see 1). Isn't (5) it so (6)?
hmmm.. anyways I am sensuriyan who's (7) doing pg (*) somewhere (8) in coimbatore which is in tamilnadu. Guys letz get together and learn (9) abt each other.

* Do you mean you've made a page from Coimbatore? If so, use make instead of doing, and since the page isn't only in Coimbatore, say from - it's from there.

1. Our is plural (it's always more than one person), but self is singular. The plural of self is selves, and since it has to agree with our, it's ourselves.

2. Tat is just spelled like that - I figure this is a typo, but might as well correct it.

3. V should be U in text talk and you properly.

4. Everyone means all the people. When it's separate like you had it (every one), it implies that one is standing in for an object. Ex. Every one of the trees is tall.

5. Always capitalize the first letter of a sentence :D

6. You made the subject of the previous sentence "you," not "it," so you wanna use isn't it (the whole sentence) so (like I said it is?)? If the previous sentence had "it" as its subject, you'd just say isn't it.

7. Always put an apostrophe (') in contractions :D

8. Somewhere is one word. When you make it into two (some where), it means some is standing in for an object. Ex. "Some flowers grow where it is hot. Some where it is cold."

9. Learn is just a better choice of word here.