Terrified...

[Firepower]

Level gladiator and marauder to 30, pick whichever one you feel most comfortable with at that point. At low level stages they are almost identical. Out of the 3 I would say DRK is a little more complex especially for a first timer.

It really is just as simple as:

PLD - shield lob to pull > flash for aoe enmity generation > standard combo for single target (Fast blade > Savage blade > Rage of Halone (Lv26))
WAR - tomahawk to pull > overpower for aoe enmity generation > standard combo for single target (Heavy swing > Skull sunder > Butchers block (Lv30))

The only differences is flash is a small AoE from your character and overpower is frontal cone. your secondary combo for PLD is Riot blade for MP to use flash again if you run out of MP. WAR has Maim for a straight buff to damage dealt.

From 1-15 you are generally solo anyway and can get used to ALL those above abilities before even setting foot in a dungeon so once you familiarise yourself it is easy.

From 30 onwards it really is personal preference but WAR does get the additional cushion of Defiance at 30 which gives you a much easier time holding enmity and a larger HP pool. At 40 However PLD gets Shield oath which slightly balances it.