Converting clothes from Sims 3 to Sims 4?

Personaly, I think this is a way to ambitious project to start with if you're new to meshes. You need to know basic meshing, blender as a tool (interface and management), the concept of UV mapping (and, the way EA does it for Sims 4, that is way to different that was in TS3.

Also, you need to know the textures: Normal/bump Map, Diffuses, Speculars (which are different so you need to make them from scratch), Shadow images. Understanding of LODS (Levels of details) and mesh polycount reducing.

I know it might look a little bit overwhelming but it's a process you need to learn slowly. Nobody learned how to create CC in one day. You usually start from basics and work your way up. I strongly recommend you to read the "Beginners tutorials" we have here in the forum to understand a little more of the things I listed you above. You have a fully organized INDEX of tutorials you can use to find them easily. The index is HERE. Look for the CAS section of the tutorials mostly, but all the beginners tutorials might help you anyways, it's not bad to read them. 

You should start with basic frankmeshing, that's my advice, it will give you some inside in the clothing process.

This is quite some work but I'm sure if you take your time you will achieve it. You can ask as many questions as you need when you read the tutorials. (There're many useful ones, guides for clothing and meshing, even videotutorials, so definitely take a look at those).

Good luck! If you get stuck don't hesitate to ask.

You can use my tutorial for convert sims 3 clothes to sims 4
cepzid.tumblr.com/post/158180239807/how-to-convert-sims-3-clothes-to-sims-4