1) The day that you experienced a flow of blood is the day from which you calculate your veset. However, the day that you began staining, and became niddah after inserting the tampon, is the first day for the calculation of the minimum wait.

The advice that your kallah teacher gave you was correct. Please see our pages on stains and toilet paper for further discussion. Note that we follow the position that you can disregard blood seen on toilet paper as long as you waited several seconds (ideally, about 15 seconds) after urinating and before wiping.

Once you have some spotting around the time you expect your period, we recommend refraining from actual marital relations even if you are not niddah. This is a precaution to allow you to clarify your status, not a halachic requirement (learn more here).

On the day of your veset, you need to perform a bedikah. Finding discharge of a niddah color on the bedikah cloth would render you niddah.

2) You do not have to distinguish for these purposes between premenstrual staining several days before your period, and light staining at the start of your period. When you first begin staining, you can avoid becoming niddah by being careful with the precautions of stains (such as wearing colored underwear, and waiting after urinating before wiping). When your period starts in earnest, whether quickly or after several days, you will become niddah.

Sometimes, premenstrual staining can be considered a veset haguf (a veset based on a physical symptom). This is relevant especially if the pattern is consistent and/or if the staining ends within about a day of the onset of your period. You can learn more about this here, and get back to us with any questions.