I agree, some girls just have it, no matter how they're dressed you can clearly see/sense it. But you can always fake it.
1. Good posture. Boobs to the front. Ass up.
2. Movements, mannerisms - I think slower movements appear more sexy. Play with your hair. Expose/touch your neck. Long eye contact.
3. Clothing - you can't go wrong here if you'll go for your typical sexy-considered stuff. But without going overboard, too flashy etc. You can easily find ideas on Pinterest or just googling.
4. Hair - loose and high ponytail are your cliche sexy hairstyles. Bonus if long or curls
5. Makeup - strong lipstick.
This is the easiest route. It's stereotypical, but people think in stereotypes. Force some of it on yourself, try it out, and maybe you'll find your own way of sexy out of it.