I use hardware to box rather than software, so there may be some things that the software can do which I need to use macros for but you wouldn't, but I'll try and give some examples of when ingame macros might be handy (I also suggest you read the stickies in the macros and addons section).

It's probably something you'll develop as you go along in-game, but there are some fairly standard things.

You'll probably want a /follow macro for your priest and also macros can be used to make things like quest accepting easier.

Heal-wise you might be looking for something like:
/cast [modifier:ctrl,help,target=targettarget,help] Heal

This will cast heal if the ctrl key is held down on your target if it is friendly or your targets target if that is friendly (so if your priest is targeting a mob you can still heal your rogue easily).

For dps you might want something like:
/cast [nochanneling,harm,target=targettarget,harm] Mind Flay

Which will try to cast Mind Flay on you target (if unfriendly) or you targets target, but if you're already casting Mind Flay then it won't interupt it (so you could bind it to a button that you mash on your rogue).

Or use a cast sequence:
/castsequence reset=target/combat Mind Blast, Shadow Word: Pain, Mind Flay, Mind Flay, Mind Flay

so you get one button to dps on your priest.

These were written off the top of my head, so may not work in game, it's just to give a general gist of what you might want to use macros for.

Looting wise you need to tab to loot (as it's done on a mouse click. Meaning collection quests are a pain, but for everything else you can stick it on FFA loot and grab everything with your rogue.