If you are new please start here:Basic AE
You can keyframe Source text to change the text over time in a source layer or use expressions and/or text animators, but that's more advanced and complicated than you need to get for your project.
Generally an AE comp should only covers one shot and be used for effects and graphics that cannot be recreated in a non linear editor. AE is not an editing app and if you are calling out a bunch of different items on a bunch of different shots in AE and the comp is longer than just a few seconds you are working yourself into an editing and compositing nightmare that will eat up your time. More than 90% of my AE work is a single shot under seven seconds long. You edit in Premiere Pro. In fact, Pro's titler is far easier to use and the job will be much faster if all you are doing is adding a simple text callout as you have shown in your screenshots. It will also render a lot faster.
That said, get a handle on setting layer in and out points, editing and animating transform properties (position, scale, rotation opacity). and because AE is a layer based compositing and motion graphics app not designed for editing, limit the length of your comps to a single shot for most of your work.
There is a lot to learn. For almost all effects and standard workflow questions the Search Help field at the top right corner of AE is your best friend. Type the name of the effect or the procedure like "edit text" there and check our the help files and community resources. Using AE is more like landing an F-18 on a aircraft carrier at night in a hurricane than driving a car. There are a zillion switches and procedures that you need to know to do complicated things and you will never find them without spending some serious time doing some training.