Im trying to install a mod that's in version 1.1. So I think I will need to install forge for 1.1 but when I try to install forge for 1.1 it gives me a zip folder and if I convert that into a jar file when I try to open it with java it gives me a error message. You know what to do or if it will even work?
You need to extract the file and add the contents directly to the Minecraft jar; I don't think there were any automated installers back then and even if there were they likely would not work due to changes to how the launcher handles versions.
First, you want to make a copy of the version folder for the version that you want to modify (e.g. .minecraft\versions\1.1), then rename it and the jar and json files inside to something else.
Then, you'll want to open the json file and edit the following lines (example from 1.6.4 but they all appear to have the same layout):
So they look like this; note that the entire "downloads" section has been removed and the "id" line now has the new name you gave to your custom version (in this case, TMCWv4, which is the name I gave to my own custom version):
Next, extract the files from the zip folder you downloaded and add them to the Minecraft jar and delete the META-INF folder, using a zip utility like 7Zip or WinRar (I use WinRar solely to install mods; I initially tried 7Zip but it gave me some errors, I do not know it it still has problems 5 years later but do not rename the jar to zip and open it with Windows Explorer as there are often class files with invalid filenames, at least on Windows and possibly other operating systems, which will break things).
Note that if you are installing Forge or another modloader you only do this for Forge itself, not the actual mods, which go into the mods folder as usual.
At this point you should be able to open the launcher (note - do not have the launcher open while editing the game files) and find your custom version in the version list in the profile editor; it is a good idea to create a new profile with its own game directory as otherwise the game will very likely crash if you ran a current version in the same directory (you can fix this by deleting options.txt but this will reset any settings).
One other thing to note - ModLoader is incompatible with the post-1.6 folder structure and will crash with a "URI is not hierarchical" error; the only way to fix this is to use a 3rd party launcher which can replicate the original pre-1.6 directory structure (if the mod mentions a "bin" folder this is the case), or possibly the patch here, but I do not know if it still works (it says that it can be used on versions back to Beta but only 1.2.5, 1.4.7, and 1.5.2 were tested, and that was 5 years ago).
Also, you will not be able to get any sounds, other than ones added by the mods, unless you can get a copy of the original assets from an old .minecraft installation; likewise, you have to use a texture pack if you want a custom skin, and custom heads will use the default skin unless they use mod assets.