Hi, me and my friend wants to do a custom 1 vs 1 fight between (me)singed, and alistair(my friend). I don't play singed alot so how would you win a 1 vs 1 game as singed against alistair? Just some tips and item builds would help. thx

yes Singed should face roll Alistair 1v1

Start of by mostly last hitting. Start your poison and fling him if he wants to attack you.
So your focus early on should be getting money, not killing him. (You probably could, but this strategy should be foolproof.)
Try to get an early catalyst and tier 1 boots (you should probably start with the boots and pots). You can start to bully him now and gradually increase your bullying. Zone him when you can, don't push if that prevents you from zoning.
You can farm behind his turret if you no longer fear him.
RoA is good.
FoN is good if he is building AP/tank.
Consider Void Staff if he is building a lot of MR.
Rylai's is another good item.
Consider Sorc boots if he isn't building any MR (Merc is the obvious choice on Singed generally, but neither headbutt nor pulverize is reduced by tenacity. AFAIK)

Take ignite + ghost

Alistar doesn't have high sustained dps.

Start Philo, and build WotA followed by a revolver with mana regen runes and just laugh...

thx guys, great answers, so I should focus more on ap and sustain rather then being tanky?

The only way he can beat you is if he goes tanky dps. Warmogs/atmas and maybe a wits end and he would be able to 1v1 you. Obviously you'll never see that build on an ali in a regular game because it would be idiotic, but in a 1v1 situation that would be his only real option to beat a singed. Otherwise he just doesn't do nearly enough damage to even touch you.