Hi! And welcome 
Congrats on starting a story! I know everything might sound overwhelming but I promise, it’ll get easier by time 
For splashes and overlays (for example the invitation letter) I use the free graphic program GIMP.
Some people use Photoshop, Procreate etc.
For covers - it’s best to hire an artist. They’ll look a lot better than self made ones.
There are also a lot of free artists and editors here on the forums so just look around art threads and I’m sure you’ll find someone 
You could also try doing one yourself.
Here’s a tutorial:
Overall I REALLY recommend Joseph Evans’ tutorials. They’re made for beginners and are very easy to follow.
Also, here’s a VERY helpful website with all kinds of guides, tips and script templates:
Here’s a list of other helpful threads on the Forums:
Martini’s script template: Tappable menus
HOW TO: Phone - Text messages
HOW TO: Drive into / Drive Off the scene with Character