My first new Mattress

I have this one, or one very similar to it.
http://www.samsclub.com/sams/12-night-therapy-pressure-relief-memory-foam-twin/181748.ip
It's been five or six years and I love it.

Go to Sam's Club or Costco and get one, even the cheaper mattresses are going to be better than the old thing you're using now. I know some people who spent thousands of dollars on a mattress and it's just meh to me. I'm not so picky I guess. :P

It is very difficult to really know what's good for you personally.
I used to look after homes while the people were away on holiday and got to try all kinds of beds (I'm such a little Goldilocks :P). Only after trying a bed for several nights do you really know what's good for you. Soft, medium, medium-hard, hard, or whatever the manufacturers call theirs. And then there's all the different technologies, springs, pocket springs, memory foam, cold foam, etc etc.

That being said, there are plenty of options and you don't necessarily get a better mattress by paying more. Look for reviews and tests, other people's experiences.
The biggest problem to look out for are mattresses that get lumpy or a dent in the middle, or the sides become soft.

You can spend 2000$ now and have it last 10+ years or you could spend 200$ and buy a new one every few years. Depending on the quality of course, there are so many options.
Remember, you sleep 1/3rd of your life, better make sure it's a good sleep ;)

edit: watched this? http://www.consumerreports.org/cro/mattresses/buying-guide.htm

Specialized mattress stores often have a test bed you can try. Once you lay on it, it can be adjusted for different levels of firmness and support. You can take the information from that and use it to shop around for a mattress which will fit your price range. Personally, I feel a good mattress matters... and something of respectable quality shouldn't turn into a banana bed on you and keep you going for at least 10yrs.

I recommend going to a mattress store and trying a bunch out
Then look online to see if you can find the same brand/model cheaper

As m3rc said, more expensive doesn't necessarily mean better. But, dirt cheap is usually low quality.

Best to try things out. But I took a risk and bought a "Mattress in a box" online. Does the job fine and is quite fun to unbox. I'm in Oz, but I'm sure you could find something similar in Walmart etc.

Try turning the mattress over if you haven't already - it can make a surprising difference.

As others have said, just getting any new mattress will make a world of difference. It's important to have a decent boxspring and mattress pad too

I have one that's double-stacked and... Foam-ish? It has two layers and is pretty soft albeit it makes my bed pretty high and it still falls inwards after some use. Not sure where to specifically get it but I'd assume your local mattress tore.

that's why i sleep in my sofa :3

memory foam is better than springs.
I recommend leaving the plastic on the mattress, that way you just buy a mattress pad every 2 or 3 years
also you can skip the frame and box spring and just put the mattress on the floor, helps with room insulation at the same time
btw a queen size memory foam mattress should only run about 100$ if you wait for a good sale[btw that is for about 8 inches thick]
also on the back thing you mentioned you might try laying cardboard on a flat floor and laying on your back, not really comfy if you toss and turn, but good for your back if you stay flat on your back.