Hopefully I can help you here.
Your calorie intake while TTC insn't as important as when you are in fact, pregnant. From what I took from your post, you are currently eating 1500 a day to LOSE weight, correct? This is really just fine for right now, given your current weight.
While trying to conceive, the biggest thing I can tell you, and I know it is hard, is to RELAX. Stress will make it, for some reason, less likely. Are you using an ovulation predictor kit or anything-or simply just having lots of sex?
Take vitamins, and if your vitamin does not contain folic acid, take a folic acid supplement, or drink a glass of orange juice each morning. (Having folic acid in your system whil TTC and while pregnant-it is IN your prenatal vitamins-helps against birth defects.)
Other than that-just let nature take its course. Some people get pregnant completely by accident-like all three of my children-but for other couples it simply takes longer. A few months, up to a year is not uncommon at all.
As for your calorie intake when you become pregnant-you are NOT going to be eating 1500 calories during your first trimester, and adding 300 to that in the second. This is because 1500 is your calorie intake for weight LOSS right now-not for weight maintenance. You would not be eating at a weight loss level in the first trimester of pregnancy.
Upon becoming pregnant, I would aim for 1800-2000 calories a day for the first trimester. You would add approximately 300 to that amount in the second trimester, for a total of 2100-2300 a day.
You should expect a small gain in your first trimester-anywhere from a couple to about 8 pounds-but not a real large amount. The larger gains of about 1/2 pound-1 pound a week happen at times in the second and third trimesters.
Do you have any more questions? If there is anything that you need me to explain, feel free to ask.

Good luck to you.