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Every battle has a rule set sobyou have to choose the team according but At very first league you don't have worried about the rule as they have standard rule for all starting leagues.
The cards has three categories- melee, ranged and magic.
Melee can attack from first position (until unless they don'thave any ability).
Ranged can't attack form first position (all other positions are ok).
Magic can attack form any position.

So if I have to select a team I will prefer melee at first position followed my magic and ranged cards.

If the first position card has the healing ability then it will be great as the card will be capable for healing him self after every round which will make his more stronger.

Let me tak a little about the ability- ability are special characteristics of a card which provide buff ti him self or may be debuff to opponents card. I already mentioned healing, another one is reach ability which means a Melee card with reach can attack from second position also.

To now I know in general a melee with reach is suitable for second position.

I assume you already know how to identify the card categories like melee, ranged or magic.

Discussion will help to explore more so ask quesion if you want to explore more.

Check below like to get more idea for abilities and card stats

https://splinterlands.com/?p=gameplay