> **I am junior** and I am heavily involved in some features that particular client uses or request

There's the key right there.  Since you're junior, follow the lead of the more senior staff and let them do most of the talking, because that's largely what will be expected.

The best way you can prepare is to know your subject and know the features that concern them and be prepared to answer questions that are directed at you.

Dress well (but not overdressed) and look professional. Smile often and make eye contact. Do both of these without being creepy though.  You need to look relaxed. Relaxed implies confidence and confidence is what your clients need to see.  If you're confident in your team, they will be too.