Eyup mate! Kudos for giving this a go! I remember the first gig I ever played was in front of 400 people and our singer decided to just not show up ever again. He literally quit and didn't tell us (stage fright maybe...). Bassist and Drummer both voted two to one that I sing our stuff whilst also fulfilling my duty as guitarist. So we went on as a three piece. I had never sung before, let alone sing AND play... We nailed it and I never looked back!
I haven't done both at the same time since then, though, as I'd rather give a full vocal performance than one where I am obviously struggling to juggle the two things.
So as a singer I can offer some advice here.
Endurance: Relax your torso, and kind of push out your stomach. Let the breath fill your lungs WITHOUT using your chest and lungs to do any of the work. Use your stomach only. Basically what you are doing here is letting your diaphragm expand downwards to take in oxygen. Get used to doing that for a while, and then move on to doing this first and "topping up" the rest of your lung capacity by using the chest / lungs. If you use your lungs only, the negative effects are twofold: Less vocal power, and less breath to use on long notes. Using both your diaphragm and your lungs in harmony will ensure that it takes you longer for your voice to tire, you can belt out louder notes for longer, and it naturally encourages my next tip which is.....
Sing from the GUTS.
If you can get out of the habit of using your throat to do all the vocal work early on, you will save yourself trouble of trying to re-adapt in the future when you realise you've been doing it wrong all this time. ^_^
The deeper you can summon the power and breath from, the better. It's hard to explain this bit, really, but put your hands on your stomach when you sing. The more work you can focus down there, the better. You should feel your abs region reverberating more if you get it right.
Another thing to aim for is to send your notes and your power up in a straight line from your diaphragm into the roof of the back of your mouth, as if you are trying to send the notes out through the top of your head. Again, if you are doing this right, you should be feeling the notes reverberate between your ears and in the centre of your skull.
These are all really hard things to explain in text format, especially when you're not a teacher or if you've only learned this from experience and a few lessons here and there like me, heh.
My best tip is to find a vocal coach in your area and have at least three lessons, just to get you started off on technique and directing your power. Listening to Sabaton right now, you're right, he has a very accessible range, but he also uses a very operatic / classically influenced style, so if you can find someone in your area who specialises in Classical / Operatic or even Big Band style singing (crooning a-la Sinatra), it will help you loads.
Another good thing to find is a DVD by Melissa Cross called the Zen of Screaming. She does a lot of work with metallers, and has a lot of vocal exercises in her video which will help you out loads.
Hope this helps! :)
