Cast on about 24 sts, then knit it for about 4", probably in stockinette st unless it says otherwise. Measure how many sts you get over 4" in the middle, don't include the edge sts because they're a different size and curl under. If you have more than 18 sts in 4", they're too small and you would use a larger needle; if you have less sts, they're too big and you need a smaller needle. It's also a good idea to wash the swatch like you will the finished item.
If this is for a scarf or shawl and the sts per 4" are close, that's fine, but if it's a hat or sweater that has to be a specific size, then you need to find the needle size that will get you the same gauge or it's going to be too big or too small. If it's a baby sweater, it can be a little big, they grow really fast.
sue- knitting heretic