Knitting gauge question

I am a beginner at knitting and do not understand how to get the right gauge as I live in Spain and get patterns from UK or US. The gauge is supposed to be 14sts = 4" with 5.5mm circular needles - with medium weight wool, I cannot get 4 inches with 14 stitches. I don´t know what medium weight is here in Spain! Do I use much larger needles or really thick wool like almost rug wool? Thanks for help.

Medium weight is also called worsted weight and sometimes has a 4 on the label. It’s probably the most commonly used weight.

To do a gauge swatch cast on more stitches than you need…like 30 and then knit in stockinette stitch for about 4-5 inches. Then measure how many stitches are in 4 inches.
http://knitty.com/ISSUEspring03/FEATswatchout.html
http://www.lionbrand.com/faq/7.html

Thank you for that but if I have to make more stitches to get four inches how will that affect the pattern when it says increases, won’t it make the garment much larger? I am trying to make a jersey for my grandson´s little dog. I got the pattern from Allfreeknitting and I want to use some odd wool that I have. The pattern is Ruff Knit Dog Sweater by Red Heart. I have been going mad trying all the wool I have and all my knitting needles. I should probably give up at this point!