You cant change your gender after you create your character. You can change the appearance with the in game obtainable cosmetics.
There are various way to get them you will know once you start playing the game.

Yes you can reset your specialization tree but no point doing so since you can max out the specialization tree anyway, myself and few others had already max out 3 specialization tree.
You can swap between them when you are back into the White House.

Specialization will only be unlocked after you reach lvl30 and finish the 3 faction stronghold mission.

You can progress the story at the same time but i have heard of some bug that happen well at least so far i play with my friend we can go through them together without an issue.
Its better to reach WT5 then only farm gears and weapons.

What is WT? World Tier.

Basically go like this LVL30 > WT1 >WT2 >WT3 >WT4 finally WT5.

WT5 is release today after maintenance.

Take ur time and enjoy the game reaching WT1>WT5 can be done in 1 day if you got friend who are already WT5.
No need to feel the need to rush there.