I’ll second Tracy and Lady Gene that any string can be used when its tied with intention. No need to go out and buy special string or study special knots if you don’t wish to! Tying with a goal in mind should work wonderfully 
That being said, drawing on certain correspondences can help to further guide and power up the spell, so if you’re looking for specifics, I’d recommend matching the knot and color with your goal. Here are a few ideas you might consider:
COLORS: Gold is ideal for victory and success, black for justice and “tying up” negative forces to bind them, or perhaps white for purity and a fresh start.
KNOTS: I’m not an expert, but I do like the easy Celtic Heart for matters of love or strong emotions. You might also consider a Sailor’s Knot for security, grounding, and stability.
Image from Sailor’s Knot
Just a few ideas! Whatever you choose to do, I wish you good luck in court and I hope justice prevails. Blessed be! 