League of Legends (LoL) Question: What items should support buy when his team losing at early levels?

It depends on what your team needs. If your team is heavily attack speed based, getting an early zeke's herald should help them get back into the game. If you're ridiculously far behind, a shurelyia's reverie should be good, because you can disengage, or chase down solo kills with it. Otherwise, just get an Aegis of the Legion and possibly an Oracle's Elixir.

If you have a team-fight based team, and your team is behind, make sure you get an Aegis of the Legion as quickly as possible. Decide between the jungler, sometimes the top lane, and yourself, who's going to rush this. A lot of carry junglers won't build it, but Junglers like Amumu, Maokai, and Nunu often will, so keep that in mind as well. If they have that covered, or your comp is more based around catching targets, or split pushing, get a Shurelya's Battlesong. If your team is behind, but your carry is farmed, and it's an attack speed based hyper-carry like Vayne or Kog, you could consider a Zeke's Herald. As far as I've seen so far, this will probably still be the case in season 3, though there are a couple other good options to keep your eye on, such as Kage's Last Breath, which applies a slow aura to an ally, Banner of Command, the new promote, and probably the ultimate split-push item, Mikael's Crucible (in my opinion, the best of the new support items), which is effectively a target heal and QSS on a 3 minute cooldown, and of course, Wraith Collar, which allows you to scout nearby hidden targets.

i'd say shurelya's because it allows for a large potential to engage on enemies when they're outpositioned
zeke's and aegis are standard.

generally cheaper and easily obtainable items are better so i vouch for shurely

er and yeah

buy wards.

In my opinion best buy would be Mejai's Soulstealer. You will either swing your team back into game by trading few kills to both teams couple of times without dying or you will fall of faster. As an item it has one of the best gold value compared to what you can get out of it. Still in the end it is GAMBLE. Are you willing to take the risk?

Alternative is to get something that your team benefits in teamfight such as Aegis of the Legion, which already somebody suggested. Basically any item that provides some sort of AURA is good for teamfight. Buffing your own team or DEbuffing enemy team.

All depends on your team's composition. If you have a AS attack damage carry, go with Zeke's. Shurelya's is for those without a nice gap closer or escape (like Kog'Maw) if you are behind. Aegis is for balanced offense/defense, which complements any AD carry all around well.

if you have a competent adc who you feel can carry you later in the game, keep your gp10 and buy tons of wards, make sure the map is always warded whenever you can, don't get caught by their team if they kill your wards, but vision will help your team more then anything else, auras and stuff are nice but knowing what their team is going to do allows you to win a teamfight 90% of the time if you have a competent team yourself.

Items that build off your GPs and aura items mostly. Remember to WARD ALOT. Even if they have an oracles the brief amount of sight you'll get is to important and you don't want an ally to get caught out and let the enemies snowball their advantage.

Buy a new AD-Carry

Deathcap
Void Staff
and Boots of Swiftness