Level up your characters to a certian level.For example,get Ross to LV. 10 and you can turn him into a fighter or a pirate.I know this because i'm an expert at this game.

get to level 10 then use a class changing item (note:this only works for characters who haven't changed classes)

Yes, everyone can level up at 10, but on hard (the ONLY fun mode), you want to go to level 20! That way, they are very strong. Note, Ross, Amelia, and Ewan can only go to level 10 on their 1st class (Journeyman, Pupil, and Recruit.)

to change cl***es all you have to do is use the guiding ring, ocean seal, elsian whip,
night crest, hero crest, and the master seal on the right person at level 10 or level 20
(press the r button on the item to check which item changes which unit and press the r
button on the person that you want to change to find out what cl*** they are). Note, Ross,
Amelia, and Ewan can only go to level 10 on their 1st time changing then on the next
battle they will change willingly then use the correct item to change them. Note ephraim
and eirika can only be changed by the solar and lunar bracelet

On one of the levels i found some girl named amelia, and i talked to her so she joined my
team and she is really weak. She only hits i think 3 damage, so where can i train her
because she keeps getting killed so i have to restart the chapter?

you need to get them at least over level 10 and then use a specific item on them. here's a
list:
cavalier & knight: knight crest
flying units: elysian whip
archer: orion's bolt
myrmidons, mercenaries, and fighters: hero crest
theifs & pirates: ocean seal
any magic user at all: guiding ring
eirika: solar brace
ephraim: lunar brace
ewan, ross, and amelia: just get them to level 10 and finish the battle, then start a new
battle and a cutscene will begin.
except for the three named above, you can only change jobs once

The best way to gain experience is to train. I would suggest going to Tower Valni because
it helps, but it's not too dangerous. I usually only stay on Floor 1 when I'm training,
though.

The best way to gain experience is to train. I would suggest going to Tower Valni because
it helps, but it's not too dangerous.

if you have pupil,journeyman,recruit,lvl. up to 10
Magic/Staff Wielders-GUIDING RING
Myrmidon,Mercenary,Fighter-HERO CREST
Knight,Cavalier-KNIGHT CREST
Archer-ORIONS BOLT
Wyvern rider,pegasus knight-ELYSIAN WHIP
Thief,Pirate-OCEAN SEAL
OR
use master seal coz it works on every class