Hello, I am rather new to this program and make a sketch i would like to outline with a brush like the one used in the beginning of this video https://www.youtube.com/watch?v=R5wW_65iAiU How would I get such a brush/modify some brush setting to get that same effect (strokes being thinner at the tips and thicker in the middle).

Edit: Also, is there a feature similar to the one used in the video at 4:54?

Seems to me these are vector features. Krita has vector features but I'm not too familiar with them as krita is primarily for raster painting. I'll try to see if there are similar features in Krita, but you might be better off trying Inkscape if not.

You have to tweak a default brush to behave as you desire, e.g.:

1. Select "Basic_circle":
> (main toolbar) > "Choose brush preset" > select the "Basic_circle" preset icon (it should be the 5th from top of the list)

2. Open the brush settings dialog:
> (main toolbar) > "Edit brush settings"

3. Select a Brush engine (far left):
> "Pixel"

4. Change settings (right next):
a. "Brush Tip":
- "Mask type": Default
- "Fade": 1.0
- "Spacing": Auto
b. "Size":
- "Pressure" checked
- "Low": 0%
- "High": 100%
- click on the half part of the curve, and drag the new point down (towards 0%)
c. Painting Mode:
- "Build up"

5. Save your custom brush (top of the main panel):
> "Name": "Basic_circle_tweaked" (or whatever name you prefer)
> "Save to Presets" button

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As for your second question I would use either the "Shape_fill" brush or the tool "Outline Selection Tool".
The video link you posted shows Adobe Illustrator brushes and tools, but as said above Krita is all for rasters.

Good luck!

Thanks a lot.