What Dill said.
To add, in case you don't know the difference, a regular thermometer has three numbers on it, a BBT thermometer has four and this means it's accurate at detecting the smallest temperature changes.
With OPKs, I would recommend buying the cheap strips from Amazon (or wherever else you can buy them), because they are less than a fiver for about 20 or 30. Probably best to use at least three daily, to lessen the chances of missing your lh surge, which tells you you're about to ovulate, or probably are. It's possible to ovulate as late as three days after a positive OPK but I think it's more typical to ovulate between the day of the positive and two days after. A positive must be as dark as the control line or darker.
Once you see a positive OPK, start focussing sex then and for the following days until you see a noticeable upward shift in temperature. You can have sex daily, or every other day, sperm can live for a while inside you, so it doesn't hugely matter if you miss a day. You could use a menstrual soft cup to keep his swimmers in there longer, but tbh the stuff that leaks out is more the fluid the sperm were in, rather than the sperm themselves so I read.
Tracking is good because you can ensure your luteal phase is a good length, and also see what your temps do during that time as if they drop a lot very soon after ovulating this can indicate hormonal issues. Likewise if they're unusually high in your follicular phase, before you ovulate, this can indicate an issue too. Charting can tell you a fair bit about how your body is working.
If you want to know the earliest you can use a pregnancy test then I would say 7dpo you may see something faint, but typically 7-9dpo is when implantation happens, and it takes at least two to four days after this for enough hCG to be passed into your urine for a test to detect. The problem with early testing is you're more likely to detect chemical pregnancies which are common, it's just that those who don't test early and just await af or for it to be missed, won't know. But that isn't to say every early positive results in a chemical of course! Just something to be aware of.
I really recommend using OPKs from a couple of days after af finishes if it's the first time you're using them as sometimes ovulation can take you by surprise as it did me this cycle.
Also, fertile cm is egg white or watery. If you don't see this then try drinking more water or taking evening primrose oil (doesn't work for some, however). If you have trouble still, then use a sperm friendly lube like Preseed or Conceive Plus. Don't mistake the wetness you see from arousal as fertile cm though, as these are two different things. Best way to check cm is to do it internally by putting in a clean finger, reaching up to your cervix (take not of how your cervix feels too, whether it's soft and open and high or something else), and scoop some out. If it's super stretchy then it's egg white cm
