There several other sites that people use to see what flights are available such as passrider.com. It works better than Expedia for this.
There’s no real way to secure a seat unless you pay for a ticket, since you will be flying space available. That’s the main reason why commuting sucks, because many people stress out about getting to work since it depends on a seat being available for you to get on. Many people use the jumpseat as a last resort since the paying passengers cannot ride in the jumpseat. Some airplanes have one and some have two. I have been in the situation where there were two jumpseats available but three pilots showed up to the gate and all seats in the back were taken. Good luck and welcome!