Note on your pattern instructions and check the stitch gauge required. Now, check the "weight" of the recommended yarn. Having this in mind, try to choose a yarn that is similar and from the same family of weights as the yarn that is recommended in the pattern you have chosen to make.
For example, if you are knitting something that is from the lace family, try to obtain other yarn that is similar to what is recommended from the lace family. This is usually labelled as fingering 10-count crochet thread. One thing to note is that you usually have 33 to 40 stitches (stockinette stitches) to 4 inches while generally using a 1.5 - 2.25 mm needle (0 - 1 in size). Usually, lace weight yarns are knitted on larger needles to create lacy patterns.
If you are supposed to use a sock, fingering or baby yarn, you usually have 27 - 32 stockinette stitches to 4 inches with a 2.25 - 3.25 mm knitting needle (size 1 - 3).
For a sport or a baby yarn, you usually have 23 - 26 stockinette stitches to 4 inches using a 3.25 - 3.75 mm knitting needle (size 3 to 5).
Please note that in the above examples, the number of stitches required for the 4 inches decreases each time a different yarn is used. This is a very important detail to note if you are in this above noted dilemma.
For a complete listing of the standard yarn weight system, download it from the internet ( http://en.wikipedia.org/wiki/Yarn_weight ) and you will find that the information it contains can invaluable both for the present and the future. You may find that you may have to use copy and paste into your browser in order to go to this invaluable website for the information you are seeking.
The other detail that is recommended when you are having trouble obtaining the correct gauge when you have to substitute one kind of yarn for another is to adjust your needle size. This can be easier said than done - if you are inexperienced in the knitting world, just which way should you go to achieve the results you are striving to achieve?
The best advice I can offer you is that if you have too many stitches in the required swatch you are making as compared to your instructions, you are knitting way too tight; try using larger needles or loosen your stitches. If you have fewer stitches to the inch, you are knitting too loosely; use smaller needles to achieve the desired effect.
Don't stray too far from the original needle size; it can alter the whole pattern if you happen to make this mistake.
There is a wealth of information to be found at http://learningeasyknitting.com/. Not only is there information, but it supplies encouragement as well. I find this website to be a wealth of information to the uninformed individual. When you sign up for a membership, you will receive a knitting pattern for a hooded sweatshirt. Keep up to date with the blog posts and the articles - and if you are stuck or have a question, leave a comment on the blog post that may or may not be related to the subject you want to comment on. Check it out today!