basic wordpress

I am excited to enter the world of WordPress with WPMUDEV. I am a total newbie and feeling lost. My friend Jannine who has some training and experience with WP is going to be building my WP site (actually converting my existing site into WP). She assigned me the following: choose a theme, number of website pages and then start doing layout and content. My question is, how do I decide on a theme? There are so many plus so many plugins! My site is informational and also will need to link to social media plus have e-commerce to sell products and so that people can sign up for individual appointments as well as register and pay for yoga classes. Hope that's clear!