Nissin DI866 vs Metz 50 AF-1?

Which one would be better? Which one is the better choice for a Nikon D90? Any faults? Advantages?

Planning to buy a flash soon but I don't know much about photography haha

Either of those are decent flashes and have TTL capability with Nikons. TTL is a sort of Auto flash metering - the camera & flash 'talk' to each other & the camera tells the flash what power level to fire at based on a measured 'pre-flash' exposure.

If you want the convenience of TTL - either of those would be fine. Just to add though, like most things Auto on cameras, TTL doesn't always get it right - it can vary between shots & be difficult to reproduce. I tend to use flash in manual (which these flashes can be used in as well btw), as I find it gives me more consistency.

On camera flash is very hard flat light. With these flashes you can bounce the light off walls/ceilings, etc. To soften it, but only if you've got walls/ceiling nearby & they're a neutral colour.

If you want to get into off camera flash, I suggest a set of radio triggers - RF602's are good.reliable & only £27 for a set. I also use cheaper manual flashes (as I don't use TTL) - the YN560 is good at around £60.