Yes, you can join BSc or Bachelor of Science after completing your 12th standard. This depends on your choice of stream, though. There are different courses for students in the Science stream, especially students with subjects such as Physics, Chemistry, Biology, and Mathematics. These can be as general as BSc Physics, BSc Chemistry, and BSc Biology to as specialized as BSc Computer Science, BSc Biotechnology, and BSc Environmental Science. While taking these courses requires a 12th grade Science background, different courses require different subjects as in the requirement.
If you are from a Commerce or Arts stream in school, your options may be limited for doing a BSc. In many universities, courses such as BCA or Applied Sciences would accept students even if a student has a non-science background at school. However, most of the traditional BSc programs require you to complete your 12th in Science. Research the specific requirements of the universities or colleges that you are looking to apply to, since requirements may differ from institution to institution.