Choose science subjects that you are interested in and perform well in. Have at least two. Chemistry and Biology if you want to do life sciences (Doctor, Zoologist, Conservation biology, botany).
Physics and chemistry if you want to do physical sciences (Chemists, Physicist, engineering, etc.)
Or all three if you want, but consult your teacher for advice as well.
For all sciences it is critical to keep your maths grades above C. Most scientist today also use mathematics a lot so try to find ways to improve it.