I know the theory behind it. The problem is I always sing the same note as the other guy. got any tips to help me actually sing the interval i wanna sing?
The only help I can offer here is to just practice the part until it becomes natural. It takes time but it'll pay off in the end.
you sound like my entire family

its really just a matter of practice. try singing along with songs with distinct harmonies, and only sing the harmonies (by yourself). then work towards doing that with a friend. eventually it will come naturally.

EDIT: just noticed, UG's two mintstrels just gave you advice
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Learn how to play the harmony part on an instrument and sing along with it. Eventually you'll get used to hearing your part and be able to sing it without playing it.
Or similar.... try this. Play the chords on the guitar. Instead of singing the lead vocal, practice singing the harmony vocal instead. If you can sing the harmony vocal over top of the chords, when you get to practice, all you'll need to do is just what you practiced. If your singer is singing in tune, then it will only help you that that note is there.

Could I get some more talent in the monitors, please?

I know it sounds crazy, but try to learn to inhale your voice. www.thebelcantotechnique.com

Chris is the king of relating music things to other objects in real life.
I agree with Meurglys3. Write the harmony on another instrument, then practice singing that harmony over the chord (s).
Learn the vocal line on guitar, then the harmony above or below it, and sing it along with only the harmony on guitar?

EDIT: Oh. It appears I was beaten to the punch. Nevertheless...
~don't finkdinkle when ur supposed to be dimpdickin~
One word for you: Oldies. Pop out the 80s rock and all that kind of music. There are a lot of sustained long notes that you can find the pitch to. When you're starting out singing harmony, find a note that the lead hits a lot, hit that note, then rise to the third or fifth, whichever sounds better. You know the theory like you said. Hold that note for the duration. Being able to listen to a song once then harmonize with it is a skill that takes a lot of practice.