Discuss your concerns with your teacher. Make some time before or after class to discuss how you would like to receive extra help. Your teacher may explain the problems to you one-by-one or have you stay for an afterschool class.
Create a study group. Get together with your classmates and discuss any problems you have difficulty with. A child who is not struggling may offer to help explain things easily. A study group can also be an inexpensive alternative to hiring a tutor.
Ask your school's administration office to see if they offer tutoring. They may offer free tutoring services to students who are struggling in science and math.
Look for tutoring services in your area. You will have to pay for this option, since the services are outside of your school. Examples include Tutor Vista, Tutors Teach and Interactive Math Tutor.