Sorry about all the abbreviations and acronyms... after a while they just fly off your fingers. Here's a link to familiarize yourself with them: http://www.mothering.com/community/wiki/abbreviations-and-acronyms
As Traci, said, charting is based on basal body temperature and cervical fluid observations. It is much more precise than using a calendar as when you ovulate can vary drastically and your luteal phase may be varying right now as well. Again, whether you decide to wean your ds or not is very personal choice. I would suggest starting to chart and see what you get for a few cycles. If you are ovulating and have a long enough LP to sustain a pregnancy then it is unlikely that weaning will affect your cycles much and therefore not be necessary for conception. I got pregnant when my LO was just 14 months and he ended up weaning at 17 months b/c my milk had dried up and nursing was excruciating. I was relieved, but sad as it felt too young to me as well.
So, to get going with charting, check out the information below. I typed it out for another MDC member a while back. Hopefully it will help you out as well.
Start as soon as you can- the first day of red blood is cycle day 1. Pick up a copy of Taking Charge of Your Fertility by Toni Weschler at the library (or buy it). You don't need to read it cover to cover to get started, but it really is an excellent resource.
You can pick up a basal body thermometer (BBT) at your local drug store. Some are better than others. I prefer ones that take longer than 30 seconds (they seem more accurate to me) and can hold two temperatures in memory (this is especially handy when you wake up, temp, and go back to sleep- you can look at and record the temp later). The Walgreens thermometer gets generally bad reviews. BD gets good reviews.
When temping you want to focus on a few things: taking it at the same time each morning (within 30 minutes); taking it after a solid block of sleep (3 hours is usually recommended); and making sure that you don't get out of bed, sit up, drink water, or fall asleep before or while temping. So basically you just wake up, roll over, temp, and go back to sleep or get up.
Some women are really sensitive to changes in sleep patterns or ambient temperature (like if your room is hot one morning and not the next or you wake up snuggled under a warm blanket and the next are out of the covers). But other women find that they can see their pattern despite numerous night wakings, not getting a 3 hour block of sleep prior to temping, or environmental changes. Other things that can affect temp- pretty much the same things that can delay ovulation: travel, stress, diet changes, injury, illness, and strenuous exercise. To help keep your temps more consistent you can try vaginal temping- it works the same way as oral temping, but some women get better results. Also, if your wake time varies quite a bit, you can try setting an alarm for a time in the morning when you are likely to get a decent amount of sleep just prior to it... maybe 5 or 6 in the morning? Just temp and go back to sleep until you need to wake up.
Next, cervical fluid (CF) is something you can observe throughout the day- whenever you feel it, whenever you see it (while wiping, on your panties), etc. Here is my general guide to CF:
Dry: you'll feel dry all day and there is nothing on your panties
Sticky: you'll generally feel dry and there will be a bit of staining- usually yellow- on your panties. It might be a bit crumbly or clumpy.
Creamy: generally you'll feel a little wet, lotiony in texture, could be slippery when you wipe, often it will stand up on your panties
Wet: you'll feel wet and your panties will likely be wet- maybe even soak through (I rarely get this)
Eggwhite: you might actually feel it slipping out of you throughout the day and it might fall out of you while using the bathroom, stretchy- it may stretch from you to the TP for several inches- and usually clear or yellow tinted, often quite profuse
Temp and CF are the two basic fertility signs you need to chart. There are other signs you can record like cervical position (CP), but it's not necessary unless your temps and CF are not showing a clear pattern.
So what you're going to see is a group of low temps from the start of AF, which is cycle day 1 (CD 1) until the day after ovulation (O) when you should see a spike in temp or steady climb to a higher group of temps. The days between O and AF are called your luteal phase (LP). Generally it is said that you need at least 10 days to sustain a pregnancy. The average length is 12-14 days. Your LP will not vary much- usually just by one or two days cycle to cycle. But your O day could vary considerably. So when you see O on your chart you can then predict when AF should show based on your average LP length. If you go 3 days past your longest LP length then that is an excellent sign of pregnancy. If you get 18 high temps after O that is 99% accurate- just like an HPT.
Likewise your CF will go from dry just after AF to sticky, to creamy, to EW as O approaches. You may or may not go through all the different types of CF prior to O or you could skip from dry to creamy or dry to EW. You might have many days of creamy and one or two of EW or you might have 3 or 4 days of EW with hardly any creamy. It all depends on the woman and that particular cycle. But after O you will dry up. It could be instantaneous- like the day of the spike you're dry. Or it could decrease slower- EW to creamy to stick to dry. But usually you will dry up within a few days and then may or may not experience a few more days of creamy or even wet/EW later in your LP, just prior to AF.
Anyway, that should get you started! Post if you have questions and use www.fertilityfriend.com to share your chart! Good luck!