I'm a size 36D in bras, I wanna get a sports bra but i don't know what size. Small medium or large ?

You have to try them on. Sizing varies from brand to brand, it is just a guideline. The only way to make sure it will be comfortable is to try it on. Remember to wash any sports bras you buy before wearing them.

You would be a large. Check with sales lady. Different brands run different sizes.

I don't know ive never done a sports bra in my life

To be one the safer side I would find a company that sells bra's by showing the sizes as A,B,C or D plus a number so you really know. Other than that try sports check, Nike.

if the seller does not have a size chart, they are online

Read the tags. Every manufacturer is a little different. Make sure you buy one that suits your level of activity. You would be a medium or large.

Large

Each brand is different. Some brands offer cup and band sizes too.

Try a medium or large, or try a cup/band size that is similar to what you wear now.

How do we know??? You have to go try them on, yourself, to get the right size.