B.Tech

Some of the premier engineering colleges in Tamil Nadu include B.Tech among various engineering programmes. The following is a shortlist of such institutions:

Indian Institute of Technology Madras (IIT Madras):

IIT Madras is always hailed as one of the best engineering colleges in India as it has fabulous infrastructure and some of the world's best faculty and research opportunities. This institute provides a whole range of B.Tech Programs. 
Ranking: Consistently under top 5 IITs and also overall best top engineering colleges in India.

Anna University, Chennai:

This is one of the largest technical universities for various programs at undergraduate and postgraduate levels. As far as research output and ties to the industry are concerned, Anna University is reputed.
Ranking: Credited to be at the forefront in both national and international rankings in engineering education.

Vellore Institute of Technology (VIT University):

VIT Vellore lends quality education, a fully equipped campus, and placement records to its name. An entrance exam called the VITEEE for B.Tech is conducted by it.
Ranking: Under the top private engineering colleges in the country.

PSG Tech, Coimbatore:

PSG College of Technology has developed a good reputation with the nation regarding engineering, industry collaboration, and placement. Various UG courses in engineering are offered. 
Ranking: Consistently ranks very much among the topmost Tamil Nadu colleges in engineering.

SRM Institute of Science and Technology, Kattankulathur:

SRM IST is one of the major private institutions with programs in B.Tech in almost all disciplines. This institution has state-of-the-art campus facilities and a good record of placements. 
Ranking: Best fit among India's top private institutions offering engineering.

Yes, many online learning sites offer courses in Data Science and Cybersecurity especially for the second-year B.Tech students. Noticeably, sites such as Coursera, edX, Udacity, and LinkedIn Learning have specialised programs in these areas. You will get various courses from beginner to expert level on Data Science, usually with projects hands-on- done by renowned schools like Stanford, MIT, or IBM. Courses in Cybersecurity include Cybrary, Udemy, and Coursera, with a focus ranging from introductory aspects to advanced topics such as ethical hacking, network security, and cryptography, respectively. These courses can be flexible enough to be incorporated with respect to regular studies. 

Normally, you need to have PCM (Physics, Chemistry, Mathematics) as your part of your class 12th study to get into a B.Tech programme in Computer Science Engineering (CSE). As you have finished your 12th with PCB (Physics, Chemistry, Biology), though, you may not be eligible for direct admission into a B.Tech CSE programme for many institutions.

University-specific criteria, however, may allow enrollment into the B.Tech CSE programme for other ways for students who completed their plus two with a PCB background. You may probably be suited to apply for:

B.Tech in related branches, which is Bioinformatics, Biomedical Engineering, or Software Engineering-these may have more lenient requirements for admission.
Bridge courses or foundation programs: Some institutions provide preparatory courses for students of PCB students to catch up to some of the necessary mathematics and other technical knowledge for CSE.
Direct Admission: Several private universities or colleges may accept the students from PCB stream into B.Tech CSE based on their entrance examinations or other specific criterions. It would be wise to check with the individual institutions regarding their admission process and eligibility.
And given your 12th percentage, which you say is 64, you might still have a chance of getting into one of the institutions that offer flexibility or different forms of special pathways for less traditional backgrounds. Confirm this by checking specific universities that provide admission criteria for PCB students.