this is my first time here and i am new in the bike sport. I have a mtb for a month now and i usually use it for road trips with some dirt intervals. It has a 7 speed shimano cassette (14-34) and on the road it seems too light now. I would like to change it to an 8 or 9 speed cassette with 11-30 or 11-32 assembly. Is this possible with my current hub or will i need to replace the whole rear assembly? Of course i will also change the shifter. Thank you all in advance
You don't need to change the whole hub, but you might well find it easier to do so. You will need a new freehub body.
The new hub assembly might be wider - http://sheldonbrown.com/frame-spacing.html#standard - in which case you will need to redish the wheel to get it central again. You can probably get away with using the same axle, as also described on that page, but you might need a new quick release skewer.
And you may need to spring the dropouts apart by 5mm. If you don't have a steel frame, and it's 130mm at the moment, give up and live with the 7 speed. http://sheldonbrown.com/frame-spacing.html#frameadjustment (or use a road hub, I guess).
EDIT: heltonbiker makes a good point about freehub vs. freewheel. I'd been considering upgrading my own bike which has a 7 speed cassette on freehub, and forgotten that a newer 7 speed might well be a freewheel, even though your question does say cassette. See http://sheldonbrown.com/free-k7.html (And even if you really mean cassette, someone else searching might have a freewheel.)
I've recently upgraded my wife's bike from 7 to 9. I replaced rear hub (old hub had threaded freewheel, not cassette), chain, cassette, rear mech and shifter. Also replaced gear cables (as inner and outer) but that might not be required. It is likely that you'll need to do all of that. Even if your hub has 7sp cassette mount, it is unlikely to accommodate 9sp cassette, as they have different width.
Quite a radical upgrade, but well worth it!