Owk, after downloading the app from play-store or getting it from a friend, open the app and create accout with your steemit username. For you to be able to send message, you'll be asked to verify email.
You can join any server (at the home page on the left) of your choice based on your niche e.g https://discord.gg/Y48PZshhttps://discord.gg/Y48PZsh (for Nigeria) and connect with friends to get more.