Hi, its great to hear you want to get your piggie a better cage and a companion! Inappropriate housing and social isolation can both cause a variety of health and welfare problems - so a large cage and a friend are vital for your guinea pig's well being.
I would definately recommend making a c+c cage using storage cubes. I have used store bought cages, wood and mesh home made pens, and now a c+c and I will never go back!
Pet store cages are all too small - I think there is only one cage available that even meets the minimum requirements for a single pig, let alone two. A c+c cage is much better (you can make it is big as you like as long as it meets the minimums) and is far cheaper than a commercial cage. The open top also makes them easier to clean and to catch and handle your guinea pigs.
A pair of female piggies should have a 2x4 c+c (2x3 is the bare minimum) - this is in grids not feet! But bigger is always better so try to build the largest cage you have room for - your pigs will really appreciate it.
The base of the cage is usually made from coroplast (also called corflute, correx, or just corrugated plastic depending on where you live). This is similar to normal corrugated cardboard, except it is plastic so can be properly cleaned. It makes the best base because it is lightweight and fairly easy to cut and score. You should be able to find coroplast in sign shops, or online. (I got mine from ebay).
If you can't find coroplast there are alternatives - lino is pretty common and usually easy to find.
For bedding you can use loose bedding, such as aspen or carefresh. Please do NOT use cedar or pine shavings as these are toxic and cause respiratory and liver disease.
I personally use the fleece method. You don't actually need to litter train your pigs for this - which is handy as most pigs won't litter train!
Fleece is fantastic, but you need to be careful to do it exactly right otherwise it will be gross. You have a layer of polyester fleece on top, and a thick layer of an absorbant fabric (such as towels or a mattress pad) underneath. Fleece wicks moisture, so the pee soaks down through the fleece and is absorbed by the bottom layer. The fleece dries very quickly so the pigs are not sitting in a wet patch. Obviously the poops will sit on top of the fleece so you must hoover or sweep them up daily. Every few days you just remove any last poops and throw all the fabric in the wash.
Grass hay (eg timothy hay or bluegrass) must be available at all times, in unlimited amounts. Basically if your pigs are finishing it then you need to give them more.
You can give hay loose on the floor of the cage, as most pigs do like to rustle around in it - I wouldn't recommend this if you use fleece though! The other option is to use a hay rack. Many pet store hay racks are too small and need to be refilled frequently, but you can make your own using leftover storage cubes.
It doesn't really matter where in the cage you put the bowl and bottle as long as the pigs can reach them!
As well as quality pellets (Oxbow or Kleenmama's) guinea pigs must have a constant supply of hay, and about one cup of fresh veggies per pig per day. Good veggies include lettuce (NOT iceburg), bell pepper, zucchini and cilantro. Fruit can be given in small quantities up to twice a week - apple, pear, kiwi and strawberry are good. Do NOT feed potato, onion, banana, nuts or seeds, or any treats or foods containing animal products or dairy.
Hay = unlimited. Veggies - one cup per day. Pellets = about 1/4 cup per day.
You can buy a litter tray designed for cats or small animals, or you can make one using coroplast. Any safe bedding is ok for litter (aspen or carefresh). Just be warned that litter training pigs is difficult and often impossible.
Inside the cage you need at least one hidey per pig. These can be plastic igloos (aka pigloos!), cardboard boxes etc. Pigs may also like cardboard tubes etc to chew on, tunnels to run through, cozies, cuddle cups, hammocks, paper bags stuffed with hay....
As for getting a second guinea pig I would first suggest you double check your current one is definately female! If she is, you could either adopt another female or a neutered male.
Guinea pigs should be introduced on neutral territory. As you want to make a new cage anyway this is fairly easy - you can set up the caeg and put the two pigs together in it for the first time. This way neither pig will need to defend its territory.
For the best guinea pig info check out these links.
Guinealynx is fantastic for anything to do with piggie health. GPCexplains how to build (or buy) a c+c, has a great photo gallery for ideas, and the forum contains "sticky threads" which will give you the best info on everything from diet to using the fleece method.
http://www.guineapigcages.com/
http://www.guinealynx.info/
Good luck!