Some notes on substituting yarn...
With a few points kept in mind, it is not difficult to find suitable substitute yarns for most knitting projects.
Tension
Avoid tension headaches by making a note of the tension of the yarn used in the project. Note, this is different to the tension stated in the project itself - for example, a project may use a 4ply yarn knitted at a very loose tension. You need to find the 'stated' tension (usually printed on the ball band). If the yarn used in the original design has a stated tension of 18sts/24rows over 10cm, and you can find another yarn with the same tension, that is a fairly good indicator that you may be on your way to finding a suitable substitute. While you are investigating tension, you may as well make a note of the 'weight' of the original yarn (ie 4ply, DK, aran, chunky).
Fibre
You know what tension you need, and what weight of yarn. It's also worth considering the fibre blend. If possible, finding a yarn that has a similar fibre blend is one way of ensuring that the fabric you knit behaves in a similar way to the fabric the designer created. For example, substituting an aran weight 100% silk for an aran weight 100% wool will give very different results because the two fibres have very different attributes.
Yardage
How much yarn do you need? Go back to the original pattern and you will find the number of skeins or balls of yarn to complete the project, and likely, also, the number of metres/yards of yarn per ball. Multiply the one by the other will give you a total yardage. If you know how many metres/yards are in a ball of your substitute yarn, it is simple maths to work out how many you need.
Swatch
Whether you're substituting yarn or not, you can't skip this stage. It's the only way to check that your tension matches the tension given in the pattern - if it doesn't, you won't knit a garment that's the same size as given in the pattern's measurements.