Can you get braces if you have weak teeth?

Maybe. What are weak teeth? Has your enamel been determined to have reduced hardness or thickness for some genetic reason? Best thing to do is present to an orthodontic specialist for an opinion.
Define weak teeth. Teeth can always be moved. If the teeth are physically weak - then yes - using an alternative like invisalign might work. If they are weak due to periodontal disease then your orthodontist (braces doc) will need to work with a periodontist (gum/bone doc) to ensure a successful outcome.
Yes. Braces can be worn regardless of 'weak teeth'. You should see your dentist if you have other questions.