I suggest installing the Map Editor by @Guadmaz or the Menyoo mod if you haven't already done so. With these, you can use most map mods from the "Maps" section. This is a quick and easy way to have fun.
Secondly, replacements are something I have spent all year with, to the point my game is now a whopping 127GB (excluding "duplicates" in the Mods folder).
For this, as @stillhere suggested, you need to download OpenIV, a very useful tool for both GTA V and GTA V to Import and/or export files, add/replace files!
I for one have installed many replacement map mods. Replacing billboards, house interiors, gas stations, vending machines, road textures, certain building textures, apartment interiors, etc. These are very easy to install, they in most cases just consist of literally dragging and dropping into the right folders.
Same applies to weapon and clothing replacements.
There are three problems, however, with this:
they're often very tedious.
You must never forget to make backups.
You can often be faced with crashes, and identifying which texture/prop in which location is causing the problem can be difficult and agonizing. This is why I would suggest only installing one replacement mod at a time.
I'd start with these types of replacement mods first. They're more straight-forward.
Vehicle replacement mods, while most common, are harder to achieve. People make the mistake of diving straight into the vehicle mods too soon. They think it's all about replacing files, but that's not always the case. One often may need to edit gameconfig.xml, handling files, etc. This should be, once you understand the previous types of replacement mods, probably in last case (generally speaking - some are very simple).
Lastly, add-on mods are very simple. However, they involve a lot of importing/exporting, so I would only suggest doing these when you dominate, or at least understand how to import/export files, create new files/folders and edit them correctly. Once you understand how these mods work, it's repetitive, and then it because easy, even spontaneous. It takes practice. You will be working with the GameDirectory\Update\Update.rpf file, essentially. Files like gameconfig.xml, dlclist.xml, gamneconfig.xml, extratitleupdatedata.meta, handling.meta files, found within the update.rpf\common\data; and folders like dlc_patch within update.rpf.
Please do keep in mind just how necessary it is for one to make BACKUPS whenever they decide the edit any file. Also make a complete backup of the game. This is recommended so that if the game decides to update itself, which is a very regular thing, you will be prepared!
Now, I spent a long time trying to help you out, so please show appreciation and up-vote my comment. I can tell if you do (appears in my notifications section) and I will greatly appreciate it. If you need anything at all, I'm here!
Thanks once again,