Help with my project

Hi guys, now I sit more than five hours in front of my pc but I did not come on.
I would realize an indirect ilumination under my bed. It should be switched on with a presence detector. When it realize something, the led stripe should dim in ca. 3 seconds up until the maximum analog value of 255, stay switched on for 15 seconds and then it should dim down until it is witched of.
For this that I am a rookie I would need some help. My problem is my understanding where I could add which part of the sketch. For testing I use a normal button and a led. Later the led strip, I would also switch with the arduino but than only the base of a transitor which switch the operating circuit as fast on and off how I need to dim the led strip with the pwm-signal.
Until now I found out that I have to use something with the command millis....but no more. And I never heard somethink about this. Millis because the programm sould not stay while the program is working. so if I use delay.
Is there someone which is as friendly to help me on.
Kind regard

At the top of the forum you've just posted in there's a thread called "Using millis() for timing. A beginners guide". If you're a beginner and want to learn how to use millis() for timing that's a good place to start.