16/12 generally refers to tubing with outer diameter (OD) 16mm, and inner diameter (ID) 12mm. First thing with fittings and tubing though is deciding what type of tubing you want to use - hard or soft.
Soft tubing - you guessed it, your regular bendy tubing. You can get lots of different sizes, you just need to make sure the OD/ID measurements of your fittings match. Normally used with barb compression fittings like these -
https://www.overclockers.co.uk/ek-water-blocks-ek-acf-fitting-13-19mm-elox-black-wc-765-ek.html.
Hard tubing - not really for beginners, but the two most common types are PETG and acrylic. I'd recommend PETG as it is less brittle, and generally easier to use for a beginner. You'll need a heat gun to bend the tubing. Fittings are slightly different to soft tubing, in that there are no barbs -
https://www.overclockers.co.uk/bits...enhance-multi-link-for-od-16mm-wc-636-bp.html.
Really what type of tubing/fittings you use is completely upto you. Most important thing is to use the right size and type of fitting for your tubes. Hard tubing definitely looks cleaner, but is more work in measuring out the proper lengths and doing the bends properly.
