First, go here
https://multimc.org/#Download
once you have MultiMC installed and launched, click Settings, then click Java.
Click auto-detect, this should find your Java installation.
If not, install Java, and find the install location.
Click the dots on the right, navigate to the Java installation, look for "javaw.exe".
If you have known extensions hidden, as I recall the default being, it may simply show up as "javaw".
(You'll really want to change that setting anyways. Google should bring up a guide for that.)
Copy-paste this bit into the JVM arguments text box:
-XX:+UseConcMarkSweepGC -XX:+CMSIncrementalMode -XX:-UseAdaptiveSizePolicy
Set the memory to the mod pack author's recommendation.
Always set the minimum and maximum memory allocation to the exact same number. Already enough nonsense going on in the JVM's garbage collector, don't want additional chaos from resizing.
Rule of thumb: 3-6GB for a 1.12 modpack, depending on size.
1.7.10 modpacks use significantly less RAM.
Make sure you leave at least a gigabyte of RAM for your system - 1.5 or 2 gigs left would be better.
If you can't leave 1-2 gigs for your system and also allocate enough RAM for the pack, look for another, smaller, modpack.
Speaking of which:
https://www.curseforge.com/minecraft/modpacks
Look for a pack, check the description in the page to see if it's to your liking.
I recommend *pretty much anything other than RLCraft*.
It's truly garbage, trust me.
It's not really difficult, and is not realistic at all.
(A TFC-based pack would be much closer, though even there the level of realism is curbed for the sake of gameplay. But I wouldn't recommend it as an intro to modded Minecraft. Not only is it rather difficult for a new player, it plays extremely differently from the rest of modded Minecraft, hardly representative of the scene.)
Back to the page where you found the modpack, right-click the small purple Twitch icon in the bottom-right of the modpack's entry. Copy the link.
In MultiMC, click "Add Instance" in the top left.
Click Twitch URL, paste the link, hit Check.
If this doesn't work, you can do a google search for where someone has explained how to get a .zip file of the modpack, then use the import from .zip option.
When you start a world, press F3 and look in the top right corner. The percentage of memory will flicker up and down, it should go down to 60% or less at the low end.
Modify your memory settings accordingly the next time you launch your pack.
Don't set too little. But also don't set too much.
When that dip happens, the garbage collection program has to sift through all of that excess data, which can cause heavy lagspikes. If there is not enough memory allocated, it will have to do these collections more often, which is also bad.