Creatine Monohydrate
The most-researched supplement in sports performance, in its plainest useful form: unflavoured micronised creatine monohydrate, 5 g a scoop, nothing else in the tub.
Creatine monohydrate is the reference form the large performance literature is built on, and it works by helping the phosphocreatine system regenerate ATP when your muscles are working hard. This Designs for Sport version is NSF Certified for Sport, meaning tested lots are screened for banned substances and checked against their label, which matters if you compete under testing.
Stir one scoop into water or a shake daily. There is no need to load, and timing is flexible, so pick a moment you will remember. The 450 g tub lasts about three months at a scoop a day.
For where creatine sits on thinking as well as training, see the Focus and Cognition guide.