.tmux.conf sample file

create ~/.tmux.conf and add the following content: 

# Use Ctrl-a instead of Ctrl-b as the prefix
set -g prefix C-a
unbind C-b
bind C-a send-prefix

# Enable mouse support
set -g mouse on

# Use vi-style keys in copy mode
setw -g mode-keys vi

# Renumber windows automatically
set -g renumber-windows on

# Start window numbering at 1
set -g base-index 1
setw -g pane-base-index 1

related topics

tmux Cheat Sheet — the tmux basics this config builds on.
.vimrc Reference — the same idea applied to vim's config file.
.gitconfig Reference — another dotfile worth customizing early.