WebMD has a fitness & calorie calculator - that supposedly helps you determine what your caloric intake should be to meet your weight goals.
The government has a website with info http://www.mypyramid.gov/index.html
So much depends on current weight, exercise level, some people have better metabolisms (this might be due to age, I know I had trouble putting weight on until I was in maybe my 30's, I could eat anything).
Bottom line - if you keep track of what you're currently eating, you can figure out its calorie counts. And to gain weight you need to up that.
In general a balanced diet is best of course - hopefully your doctor / nutritionist gave you some advice about calories & dietary components - whether you need more protein, or more fruits & veggies for their vitamin & mineral content, etc. (Don't do too much fat-free unless recommended by your diet - many vitamins are fat soluble, so without fat in your diet you aren't able to absorb them properly.)
If you look at the nutrition labeling on the back of almost any package of food etc., it uses a standard of 2,000 calories per day. Women probably should have fewer calories than men, in general.
Good luck, and I hope you don't get too focused on calorie counting..... IMO it might be easier, and less stressful, for you to try to sneak in extra food without focusing on the calorie content - eating many small meals a day for instance, or being sure to have a healthy snack between meals.