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 the talking, because that's 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 any questions that might be directed at you.
Dress well (but not overdressed) and look professional. Smile often. Do 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.