Gym

Anyone can help me what the most one to be consisten to upgrade?

Strength 

Defense 

Speed 

Or Dex

Thanks 

At lower levels it doesn't matter too much, but balancing them all out has always been fine if not optimal for newer accounts.

Eventually if you want to be spicy, you can choose to prioritize strength/speed for offense or defense/dex for defense. You may start winning against folks that you couldn't beat before, but you'll also start losing to others that you would be able to beat with balanced stats. So you decide.

Every gym will provide a multiplier to each skill that's unique to that gym (the green 'pips' that you see when selecting a gym).

Since this multiplier tends to go up with each new gym unlocked, its generally recommended to train in whatever stat your highest level gym specializes in (provides the largest multiplier).  Of course, if you want to just train every stat evenly - you can do that too.  Its not as efficient, but you won't see much difference in the long run.   All the gains you make at this stage are miniscule by comparison to what you will earn later - so no need to overstress about being super optimal.  

train all stats for starter

you will figured it yourself what stats you want to focus on later

Keep them balanced until 1 billion in each stat. Try to focus on stats with higher steadfast bonus from faction and better gym score.

Once you get 1 billion in each stat, it would be good to focus on either DEF or DEX and only train 3 stats using special gyms.