It can be challenging to successfully use NFP even when a regular 28-day cycle is present, but a basic understanding of how things work under "normal" circumstances is helpful even if you're not there yet. Someday you will be. In the most basic terms, here's what happens: bleeding starts, shedding the lining of the uterus so a new one can be "built." This usually takes about a week, sometimes a little less. On day 14, an egg is released from an ovary. If conception does not take place, the egg dies within 24 hours and 14 days later bleeding occurs, starting the next cycle. NFP involves the observation of fertility indicators and either abstaining from sex to avoid pregnancy or using that information to achieve pregnancy.
In greater detail:
The Follicular Phase
This is the first day of menstruation, the day you count all other days from. Normally, a woman's period lasts from 4-7 days. During this time, the body is producing Follicle Stimulating Hormone(FSH), which does exactly what it sounds like: it stimulates follicles(eggs) on an ovary. Every woman has two ovaries, but ovulation generally takes place on only one ovary at a time. During this time(bleeding), estrogen levels are at a steady low and don't fluctuate much. This is important to know if you are taking any kind of medication which affects estrogen levels(or even eating a lot of soy, which raises estrogen levels) because it will impact your cycle. On the last day of bleeding, estrogen levels begin to rise. They continue rising until they are high enough to trigger the release of Leutenizing Hormone(LH), an event called the LH Surge(the most important piece of information in NFP). The LH Surge can last 24-72 hours, and triggers ovulation. This is significant because the LH Surge can be detected using a simple at-home urine test and, in my opinion, is one of the only fool-proof indications of what your hormones are doing.
Let me stress: the LH Surge is NOT the same thing as ovulation, it precedes and triggers ovulation. These hormonal reactions are fast and sensitive; they change hourly. Once estrogen levels are high enough to trigger an LH Surge, there is no going back. The LH Surge proceeds until high enough levels of LH are reached to trigger ovulation. Within an hour of LH peaking, levels of LH begin to drop off and ovulation occurs. The decline of LH after a confirmed peak indicates ovulation has begun.
The Luteal Phase
(the portion of the menstrual cycle which begins with ovulation and ends with Day 1 of the next cycle, usually 14 days after ovulation)
Ovulation occurs. An egg is released from an ovary and is viable for roughly 24 hours. If conception does not occur, the egg dies, estrogen levels decline and in 14 days another menstrual cycle begins.
So that's what normally happens. For more in-depth information, go here.