It's not too bad, but does have it's idiosyncracies.
I've been on it for quite a few years, so not 100% recall starting from scratch.
The app is actually a bit easier to use than the web.
Bascially pick a team.
ALthough you'll have to start on Week 2 it won't be that much of a hindrance.
Have a read through this, join pick your team, and then come back and ask any questions. We can then start on double game weeks and the like!
If you're feeling really nervous you can autopick to start with.
https://www.premierleague.com/news/1252542
There is a help section too https://fantasy.premierleague.com/help
Another guide https://www.fantasyfootballpundit.com/fantasy-premier-league-beginners-guide/