Hi Sandeep, I could understand your question perfectly,
I thought this was an interesting question, so I checked my Oxford dictionary for you. It seems that I comes from Old English Ic and Old Scottish Ik. The Old German is Ih, which gives us the modern German Ich. - You will find that most basic English words are Germanic in origin because they date from the time of Old English (that is before 1000 CE). When English, German and Norse languages were all similar, being part of the same family of languages.
I don't know how the convention of writing I with a capital letter happened. I would guess it is something that came about in a later period. It would have fitted with the convention of writing proper names with an initial capital letter.
I will leave it to the etymologists among us to consider such weighty topics any further!
- Nick