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You need to first of all find out when you Ovulate in order to determine your luteal phase (the time between ovulation and your period ie the 2nd half of your cycle).

You need to learn how to chart your body signs so check out Fertility Friend.com which is free and tells you how to do this.

It involves mainly taking your BBT each morning - after ovulation your body temp rises and is consistently higher for the entire luteal phase and drops when you next get your period)

You can also use OPKs which will tell you when you are about to ovulate. Buy a batch of these online cheapies are very reliable - you'll need at least 10 the first time you chart - you start using them from about Day 10 of your cycle (Day 1 = 1st day of last period).

Checking your cervical mucus gives a good indication of your fertile window which is the time before you OV.

I'd start with Fertility Friend.

I have just recently been trying to do this as well, to determine if mine is too short and that's why I'm not getting preg. You have to chart your temp every morning at the same time. (i do 6 am) for 3 or more cycles. Your temp will be lower pre-ovulation then rise when you ovulate and stay higher for the luteal phase(which is the time from ovulation to your next period). From what I've read a normal luteal phase is 12-14 days. Then your temp will decrease and you should start your period again. If it stays high and you don't start you could be prego.

What is the luteal phase?

The luteal phase, also referred to as 'days past ovulation' or 'DPO', is the part of the cycle that starts at ovulation and ends the day before your next period. It usually lasts about 14 days and does not vary by more than a day in each person. The luteal phase is named after the corpus luteum (Latin: "yellow body"), a structure that grows on the surface of the ovary where a mature egg was released at ovulation. The corpus luteum produces progesterone in preparing the body for pregnancy. Your luteal phase must be at least 10 days long to support pregnancy.

The importance of the luteal phase

The length of the luteal phase determines the time of ovulation within your menstrual cycle. Ovulation can be delayed by a number of factors, such as stress, increased activity or medication, but the length of the luteal phase is usually constant. Taking this into account, you can calculate the time of ovulation within your cycle by subtracting the length of your luteal phase from the length of your cycle. For example, if your cycle is 28 days long and your luteal phase is 12 days long, the ovulation will occur on day 16 of your cycle (28-12=16). Ovulation Calendar uses this formula to calculate your time of ovulation.

How do I determine the length of my luteal phase?

The only way to determine the exact length of your luteal phase is through hormone-specific blood tests.

What if I don't know the length of my luteal phase?

If you are not sure about the length of your luteal phase, you can assume it is 14 days (the average length for most women-28 days).

You will have to know what day you ovulated. If you don't know this, you will not be able to figure out your luteal phase. The luteal phase is the time between ovulation and menstration.

Read Taking Charge of your Fertility by Toni Weschler

You need to know when you ovulate - your luteal phase is the number of days between ovulation and menstruation.

go get a book called:

Taking Charge of Your Fertility, 10th Anniversary Edition: The Definitive Guide to Natural Birth Control, Pregnancy Achievement, and Reproductive Health by Toni Weschler (Paperback - Oct 31, 2006

It worked for me the very first month that I tried after reading it. It helps you clear up all the myths about your cycle.

LF is how many days pass from ovulation till your next period.