We had creaky stairs then one day one of the treads dropped a couple of centimetres when I trod on it (that was a bit scary). We got a handyman in who reinforced the underside of the first eight steps with extra plywood and lots of nails. He had to take the carpet up as well so I think some of the nails went in from the top down.
If you're DIY minded you could definitely do it yourself. It's bliss now, the kids can't hear me coming up the stairs anymore so no one gets when up in the evenings.
We have a very creaky landing, it has reasonable carpet but it's very thin and I can feel the ridges of the boards underneath. We have been told that we can lift the existing carpet and replace the underlay with super thick stuff, and fix down some of the boards at the same time. Would this be possible op, before your carpet goes down?
The stairs are creaking because the wood is moving. If you screw the treads down more firmly they will stop creaking.
Can you get underneath the stairs? If so, have a look at how the stair case is constructed, and work out which bit of the stairs you can screw to the frame to sort it. If you can't you'll have to look at the way the treads (the bit you step on) rest on the risers (the vertical bits which face you as you go up) and see where you can put screws in.
It's not difficult once you work out what you're doing. Just make sure you sink the screws all the way in or they will wear through your carpet and reappear.
The same goes for floorboards elsewhere. Screw them down and they'll stop squeaking.
On the stairs yes more screws and to make really sure use some PVA glue which you could squirt/brush into any gaps. If you can see the underside of the steps, get an assistant (heavy) to walk up and down the stairs and look for movement, then apply glue and screws. Sometimes the riser (vertical) part cracks, in which case use thin plywood glued and nailed to reinforce it.