What does it mean if I have a weird smell coming from my vagina?
Vaginas are self-cleaning organs. Moisture secretions from the membranes that line the vagina are normal and help control the growth of organisms—like yeast and certain bacteria. So over time, you'll get to know what normal discharge looks and smells like. If the smell you're describing is different from your normal smell, it could be a sign of infection.
Some infections like bacterial vaginitis or yeast infections can occur organically from imbalances in the body (stress, diet, and clothing can all contribute to this). These are not STIs, and can be treated. Some of these infections may resolve on their own, but others require medical treatment. Vaginal odor can also be a sign of an STI like Chlamydia, Gonorrhea, or Trich. For this reason, it would be a good idea to see a medical provider. They can determine if an infection is present and prescribe a treatment if necessary.
What is the best way to make sure I don't smell bad before someone goes down on me?
You should not have to do anything special aside from practicing basic hygiene! Rinsing and using a mild, unscented soap around the vulva is sufficient. You should always avoid washing the vagina (the inner portion, aka the canal that leads to your uterus) with any soap. There are plenty of products out there (like wipes and certain soaps) that target people who are concerned about vaginal odor, but these can actually disrupt the pH and the normal balance of bacteria in the vagina, which can lead to infection.
You should never feel pressure from your partner to smell a certain way. Your normal, natural smell is the healthiest one.