I was wondering if anyone could explain why Gitmek is said as Gidiyor instead of Gitiyor. Also, how do you know when to change the t to a d.

Only four verbs change their root spelling from -t to -d when adding a vowel :
gitmek - to go - becomes - gidiyorum - I am going
ditmek - to shred - (this verb is often used in recipes) - becomes - didiyor - he shreds
tatmak - to taste (of) - becomes - tadıyor - it tastes (of..)
etmek - to do/perform - becomes - ediyorum - I am doing etc.
Note: This verb - etmek - includes all verbs containing - etmek such as kaybetmek - to lose and affetmek - to pardon, to excuse
All other verbs retain their original spelling, for instance:
bitmek - to end - bitiyor - it is finishing - [NOT bidiyor]
batmak - to sink - batıyor - it is sinking - [NOT badıyor]

Taken from www.turkishlanguage.co.uk

Wow! İ never knew this!

Well now you do!