Hi, I assume your using the Regular Launcher right?
A few important notes before getting started:
Note 1: Before installing Forge you need to consider what version you want to play on, 1.12.2, 1.15.2 or anything earlier. Versions are important as mods and modloaders (Forge, Fabric, and others) change and Mojang's code changes so compatibility between versions like with 1.9.4 and 1.10.2 aren't common.
Note 2: Another thing of note is that mods are all installed the same way, get Java (recommended 64bit if you have a 64bit computer, if you have 32bit running later versions isn't likely an option for 1.13+). Get Forge, find where to put mods and then if something conflicts then I'd recommend do either of the three.
1.just remove mods that could cause issues and see what happens (keep removing any until you get the game working and readd some over time to pinpoint the odd one out,
2.add library or API mods that are required (you can find these by clicking on the file and seeing the changelog and the required mods needed to run alongside it).
3.Post the crash report to the forums or to the page you downloaded the mod from's comments and see if the mod dev or someone that can read crash reports on these forums can help you with it.
Note 3: I don't recommend relying on Youtubers that have 'how to install this mod' (like many I know) but I do recommend any that say 'how to install Forge or Fabric', the modloader is important as it makes mods work with the game, adding a compatibility layer.
Installing mods at first can be confusing but as it's simple once you do it enough times it can be easy to do in about 2 minutes.
And besides if mod installations haven't changed in years (since 1.6.2 to be exact) then clearly the install tutorials for each mod seems pretty silly right? (I probably didn't need to bring that up but as someone that sees it a lot I find it misleading and a chance to get views rather than being helpful and trustworthy).
3.Next double click the jar file you just downloaded and you'll see a window like image 1 (Forge Installer). You can select where you want to install it (if you installed your launcher somewhere but the default then change this, otherwise it's best to leave it as the default if you installed Minecraft to the default location) but the thing that is important is the 'Client' part. So make sure that is selected. Then select Ok and let it do it's thing, it will download and prepare all the necessary things just like installing any other application.
4.Once it's done open your launcher (if it is already open close it then relaunch it so the Forge version profile can appear).
Now that this is done, go to %appdata% (or the Mac/Linux equivalent).
You can find this by searching for it in a Windows search bar.
If you want to find it a different way I'd recommend loading your game up and make your way to the Resource Packs menu, select the left button and it will open that folder, move out a folder and you'll be in .minecraft, now get your mods and put them into the mods folder Forge should have created if not just create a folder mods in lower case and put them in there.
5.Now launch the game access (if you have it left open it won't work so make sure you restart the game again so it can be prepared to load the mods and Forge
Now your game should be modded.
Just make sure to get mods from safe sites like Curseforge or PlanetMinecraft for example, some like 9Minecraft (and there is a list with hundreds that are fan content stealers, you can find them all here: https://stopmodreposts.org/sites.html)
Otherwise if there is anything else your curious about (or I missed it) just ask or look around the forums for tutorials in the tutorial thread.
Sorry if this post was a bit big, just thought it would be good to know as there is many things newcomers can miss or encounter that I myself see happen and might want to know.