There are a variety of ways to get mods for Minecraft, but the most common and reliable method is to download them from websites that offer a database or library of mods. The two most popular websites for this are CurseForge and MinecraftMods. Both sites offer a wide variety of mods, and usually the latest versions of mods will be available on one or both of these sites.
CurseForge is probably the more user-friendly site, with a cleaner interface and better organization. However, it should be noted that you will need to create a free account in order to download anything from CurseForge. MinecraftMods does not require an account, but the site can be more difficult to navigate due to its lack of organization.
Once you have found a mod that you want to download, simply click on the “Download” button (or equivalent) and save the file to your computer. Most mods come in the form of a .jar file, which can be added to your Minecraft game by opening up the “mods” folder found within your .minecraft directory (this folder will be created after you run Minecraft with Forge installed). Simply drag and drop the downloaded .jar file into this “mods” folder, then launchMinecraft and enjoy!