I assume you are just looking for recommendations for specific gravities of 'styles' of cider? If not, there are numerous movies on you tube that can explain how to use a hydrometer or refractometer. A movie would probably be able to explain that process better than written text.
For a one gallon batch, taking readings throughout fermentation with a hydrometer and test cylinder will soon deplete your one gallon batch. Give it at least 2 weeks before taking a sample. Depending on your recipe and yeast strain, you might even give it a month before checking. After taking a sample, no matter how clean your equipment is, I personally wouldn't return it to the batch. A refractometer would work best in this situation because it only uses a drop or two of must to determine sugar content, but also requires some calculations when alcohol is present.
Starting and finishing gravities are going to be determined by personal preferences. Are you looking to make apple wine or a lower ABV hard cider? There are a lot of variables that will determine your OG and FG. If you can let us know the style you prefer, someone might be able to assist with a recipe or additional details.