During my school and college years, I competed in many different math competitions, as I could tackle problems together with one of my best friends, Qi Zhu, which was a lot of fun!
I hope to inspire future generations by participating in these competitions too, as one will not only gain problem-solving skills but also close friendships and happy memories to look back on!
Nowadays, there are many free online resources to train for these competitions — even more than during my time. The best advice I can give is to invest plenty of time trying to solve these problems to develop a strong intuition for them, and only then look up the solution, which you should deeply understand and memorize.

Now, let me list some write-ups from a teenager who tried to learn lucid dreaming to solve problems during sleep...
Let BWM := German Federal Mathematics Competition.

• BWM Round 1 (1^st Prize)  - My solutions
• BWM Round 2 (2^nd Prize) - My solutions

• BWM Round 1 (1^st Prize) - My solutions
• BWM Round 2 (1^st Prize) - My solutions
  
• BWM Round 3: Talked with Prof. Dr. Daniel Grieser about Group Theory.

• BWM Round 1 (1^st Prize) - My solutions
• BWM Round 2 (1^st Prize) - My solutions
  
• BWM Round 3: Talked with Prof. Dr. Jakob Stix about Galois Theory and became national winner 🥇.

• BWM Round 1 (1^st Prize) - My solutions
• BWM Round 2 (1^st Prize) - My solutions
  
• BWM Round 3: Became two-time national winner 🥇.

A personal note:

• ‘BWM 2016 Round 2 - Solution 2’ is one of my favorite proofs, since it is very surprising. By assuming the finiteness of certain integers related to primes and triangular numbers, we show that 1 = 0 in 𝔽₂. Crazy!
• ‘BWM 2017 Round 2 - Problem 4’ was by far the hardest one (not just in my opinion), even though number theory was my favorite area!