I found a useful website.

Today, A friend told me about this website that can help me study about vocabulary.

I tried It and It was really useful.

explination for the correction.

I  (find) is a present tense your sentence "I find a useful website". is a past tense instead of using the word find your suppose to write Found so it would fit in the sentence I FOUND A USEFUL WEBSITE.

I found a useful website.

Today, A friend told me about this website that can help me study (more) THE WORD "MORE" does not fit in the sentence the write word that you should be using is ABOUT vocabulary. THE SENTENCE SHOULD BE : Today, A friend told me about this website that can help me study about vocabulary.

I (tryed) the word that you used "tryed" is correct but you got the spelling wrong the right spelling is TRIED. THE RIGHT WAY TO WRITE THIS IS : I TRIED IT.

(it's)does not fit in the sentence " I TRIED IT , IT'S realy useful." the right way to write it is : I Tried it and it was (realy) the word "realy" was used in the sentence right but the spelling is wrong the correct spelling is REALLY useful 

the right way to write this sentence is : I tried It and It was really useful.