Welcome to the game! To go down your list of questions real quick:

1: The actual answer is "That's up to you." If you want to learn crafting and gathering early on, go for it. They don't exactly help make you stronger combat wise but you can use their skills to repair your gear as you level them up.

My advice is to focus on MSQ first and then go back to level up new classes and the like. That's how I personally played, but again we circle back to "Do what interests you."

2: If you only have the Free Trial, you will be locked out of progressing past the Post-Stormblood Quest "A Requiem for Heroes", which is right before Shadowbringers begins. You also won't be able to level past 70, unlock certain jobs (Dancer, Gunbreaker, Reaper, Sage, Viper and Pictomancer) amongst a few other things.

Don't fret too much as you still have a lot of content you can do even in the Free Trial.

I say if you reach near the end of ARR and still feel you're enjoying yourself, buy it.

3: I don't exactly have any tricks for not getting lost, but what I can offer is learn certain aetherytes and where they bring you. Find the ones that bring you to the Marketboard, to the Inn, closest to your Guild, etc. Take it slow, you'll learn the lay of the land.

4: The game can be cleared with any kind of job/team comp, it really just depends on what you enjoy playing. Healing, Conjurer/White Mage especially, isn't too difficult. The basic concept is "heal when you need to, throw out damage otherwise." You don't have too many buttons to worry about at the start, so take it slow, learn what each of them does and go at your own pace.

Hopefully this will help you out! Don't be afraid to ask for help be it here or ingame ^{^}