How to Build an Efficient Team?

Agree with HM.  This is not WOW Raids, where you need 2 tanks an off tank, 4 healers, 10 Melee and 9 range. 

Synergy matters more.  For example, Imperial Troopers, General Veers, Adm Piett, Col Strak, Stormtrooper and Range Trooper are pretty formidable and their team is made to put out punishing damage and bonus turn meter.  The same for Phoenix and Geonosians.  Nightsisters are another very strong team that feeds off bonus turn meter and puts out rapid punishing dps.  You're more looking for faction tags, Rebels, Bounty Hunters, Imperial Remnant, etc.  Also there are some toons that work together regardless of faction.  Example Starkiller does his best work with a Sith toon, Unaligned Force User, a Jedi Tank, and a light side healer.  This makes for a really interesting team and there are several combos you can use.

You can google COUNTERS to teams on SWGOH.GG and look at what teams work to break other players as well.  It's also about reading kits and testing for yourself what works.  As mentioned Mods and Datacrons do affect game play and response.

Since this is your beginning account, there is a great video by an influencer who gives advice for farming what will really benefit you in an efficient manner to unlock Journey Toons and eventually your GLs if you don't want to spend cubits on stuff.  There is a lot of information out there, don't get frustrated or give up.  Use the internet and watch the videos of each of the influencers and see which one speaks to your play style or information uptake.  We're all here to support you and offer advice to the best of our ability as fellow players.