I want to make a mod and I already know a lot about mapping and modding. But now I got stuck. I will now list the things, which I want to know:

1: How can I make a custom Gametype
2: How can I create and import weapon models ( I've already read both tutorials about importing models, but I can't find any information on how to make custom animations and so on)
3: How can I create a texture which fits on my weapon model.
4: How do I create scripts for my mod and where do I have to place them.

I know that these are a lot of questions but the most important one is number 2. It would be nice if someone could make a video tutorial about it.

If you do not want to help me, I can live with it, but it would be really nice if someone could help me.

pace Gumpi, but i don't think you'll be able to write advanced scripts like gametypes or get custom animated models into the game if you just started modding.

2: there's currently no other way then using Maya and the official export plugins for xmodels/xanims. you have to rig (link tags and joints) to your custom model and animate the bone skeleton afterwards. then export, setup the weapon file etc.
More info: CoD2 - Rigging | CoD4 - Rigging | CoD2 - Viewsleeve Animation

3: you have to create a uv-map in Maya, export the unwrapped outline image, create a texture using any 2d image manipulation tool (e.g. Paint.NET, Photoshop, 3D Coat, ...) and load the texture into Maya to apply it. The UV-map will be exported too when it creates the XMODEL_EXPORT file. Use AssMan to convert into XMODEL (like described in the model export/import tutorial).

Thanks for the links, they are very usefull. But I also know, that I wont be able to make complicated scripts and gametypes, I just want to edit the stock ones a little bit and this should be know problem for me. So if anyone knows how to edit them, please tell me.

Just tweaking a stock gametype is a bit different to starting from scratch.

You can find the gametype scripts in raw/maps/mp/gametypes/

for example, sd.gsc is the script for Search and Destroy.

Unlike previous CoDs a lot of the generic code for the gametypes (like the bits for handling player damage) now live in _globallogic.gsc and are common to all the gametypes.

Anything in particular you're trying to do?

I want to change the s&d gametype, so that one team(attackers) can respawn and the other team (defenders) cannot. This sounds unfair, but it isn't because I am going to make maps which compensate that effect.