lets take the title, merak ediyorum da..

here the da highlights that i am doing something for a reason, ie. the verb before it. thats correct yeah?

im not sure however, whether or not da is just used as a word or as some kind of changeable suffix which follows vowel harmony..??

well? can i have some help please??

We have two smillar things in Turkish:

1. -de,-da,-te,-ta suffix:

This is the locative case suffix. It means 'in', 'at', 'on'.
Okulda, durakta, evde,

2. the conjunction 'de', 'da':

This is what you are asking. It is written always as another word. It doesn't have a 'te', 'ta' form. It has a few different meanings.

First meaning: too, also, as well:

Ali de gitti.
Ali has gone as well.

Ben de gidiyorum.
I'm going as well.

-Ben dondurma istiyorum.
-Ben de.
-I want ice cream.
-Me too.

Second meaning: Connects an action to another. This usage is smillar to 'by' and 'with'.

- Buradan atlayabilir misin?
- Hayır.
- Ben atlarım.
- İyi. Atla da gör gününü.

Hava çok soğuk. Eldivenlerini giy de git.

yah i do know this..

i dont think u know what i mean..

take the title of the thread as an example..
merak ediyorum da
here doesnt the "da" add sth. onto the sentence??

It is a conjugation. It connects the previous part of sentence to the following part. In your example It doesnt have any meaning. It just helps to build the sentence in a different way.

Merak ediyorum da, neden bu kadar çok sigara içiyorsun?
Neden bu kadar çok sigara içtiğini merak ediyorum.