I would recommend you the following algorithm:
1. Identify the companies interesting for you. Many people are interested mainly in MBB, you can start defining the exact list of companies interesting for you for the internship
2. Check the requirements and application details.
3. Start your preparation with reading Case In Point by Cosentino for a general understanding of what a consulting interview is.
4. Start learning and practicing the cases. Some you can find in Case Library and practice it with your partner or experience coach.
5. Purchase and read Viktor Cheng Book (Amazon Kindle store) and listen to LOMS
Once you feel you are not improving anymore, I would recommend you to take an expert coach for structured feedback and polishing your own performance.
Hope this helps,