How do I trade ISK with another character?

Several options exist:

If neither character is on a trial account, one can right-click the other's icon (in People & Places or anywhere else) and select Send Money.

If neither character is on a trial account, either can setup an item exchange contract which includes isk.

Regardless of whether characters are on trial accounts, they can simultaneously dock in the same station and trade with each other (by right-clicking the other character in the Guests panel.

If two trial account characters cannot dock in the same station simultaneously (e.g. they are on the same account), they can exchange isk by trying to sell a common item at a ludicrously high price, and then have the other character buy it.

Finally if you are in the same player run corp you can exchange items via the corp hangar (click on corp hangar in the station it's at, usually the HQ, drag and drop items into it. Just buy items of a good value and trade them to let the other party sell them at little loss or, if you have picked well, at a profit. Ammo is usully a good place to start.