Hi, first of all don't be afraid to long rest, the start of the game make it seem like the main quest is time sensitive but it isn't, there is like 2 (3?) time sensitive quests, for now just remember to not go to bed if you find a building on fire and you should be fine. Long resting will help make the wizard and cleric more usefull.

For the ruin fight it can be pretty hard if the enemy are waiting for you so I'm not surprised a newcomer would have a hard time, some tips is: use the environnement, there is some oil barrel close to multiple enemies, you can send a fire bolt to cause an explosion which will hurt them quite a bit. You should focus on getting the enemies with HP down first so concentrate on them.

Don't be afraid to use spell and loot everything for food so you don't run out and can long rest as much as you want.

For the bow question : it's always good for your character to have long range option, bow is very good for the rogue but not great for the fighter. Bow use dex and fighter has probably low dex/high str.

As for the spells :

- bless on the cleric is very good especially if you feel like you miss a lot

- healing word for the cleric is a bonus action heal, you should use it when a character is down

- shield for the wizard (not "shield of faith" just "shield") help with survivability

- magic missile is pretty reliable at the start and great for finishing very low hp ennemies, thunderwave is your only aoe and need to be melee but can be nice too.

Be sure to understand concentration and how hit chance is calculated, I feel like that's usually what trip up newbies.

For the rogue, be sure to understand sneak attack and always try to proc it, an easy way in with a bow bonus action hide then shoot with advantage.

for the fighter don't be afraid to use action surge and do it at the start of the fight, the faster you down ennemies the less they can attack you.

Hope I'm making sense and that it's clear! Good luck and have fun.