Communication Design

For (Design Aptitude Test), there isn't much that you can do. Firstly, and most importantly, you should have an aptitude for design. NID papers are designed in such a way that only those who have a creative side will be able to clear it. The question paper is a mix of creativity, logical thinking and general knowledge. You need to be aware of the latest happenings in the world. Cramming up yearbooks won't help you, seriously. Instead make a habit of reading the newspaper every day.
After clearing that, you'll have to prepare for the studio test and the personal interview. The studio test judges you on your material handling and of course creativity. You could be provided any material, like clay, terracotta, cardboard anything and from a set of themes, you may have to design something of your choice. Since it is time bound, you have to be real quick and efficiently use the materials provided to you.
The personal interview is where the faculty evaluates you in person. They may ask you about your hobbies, interests and even talent. They may even ask you to sing if you're a singer. If you have a history of artwork, you get to showcase it to them. Whatever you have done till now, be it a random sketch or doodle, bring it along. But make sure it's your own work otherwise you may get caught and it's not that difficult for them to do so. Make sure that your portfolio stands out. It should speak for you and define you. Don't try to include or steal somebody else's ideas. Be unique and come up with something from your side. Follow your instinct.
For all this, you can join some coaching institute which will help you ENHANCE your creative side. There are a lot of coaching institutes like BRDS, Skyblue, Silica, Pahal Design and the list goes on. But I would suggest you go ahead with BRDS because they have the maximum selections every year and also the material provided by them is good. Even if you don't wish to join any coaching institute, that's okay. I have friends who have cleared NID without joining any coaching and who even after joining such institutes haven't cleared any of the design entrances.
Lastly, don't stress about the exam much! Stress makes you think less creatively and that is something you wouldn't want to happen on the day of exam. For the interview, don't worry at all. Be confident and be yourself.