Put together a schematic of what you have built. Show all power sources.
For Low side switching, we'll use a logic level N channel MOSFET for the convenience. Logic Level MOSFETs operate within the range of 0-3.3 to 5 V range where we can easily operate using a microcontroller directly. See the operating Voltage (Vgs) is 3.84 V where it can be operated using even an Arduino.
Note the pull down resistor R3 is on the processor side of the Gate resistor R1. There are a lot of bad drawings showing the pull down resistor on the MOSFET gate, this forms a divider lowering the gate drive voltage.
There are different kinds of LED strips... They are usually easier to use than individual "high power" LEDs (1W and up) because regular LEDs want a constant current source and strips are usually voltage-controlled (with current control/limiting built-in). But some are not dimmable. But you can make anything flicker, if that's what you want.