Typically, use a shield if your opponent is using a weapon that you don't learn. Or if they are hasted. Or if they are just hitting you too much. Or if your the tank while your ranking.

Use dual wield where the previous conditions don't apply. You can also get away with dual wielding sometimes if your hasted and they are not.
Or if your target is (or has) low hps and you want to hit them as much as possible to kill them quickly, ie: your about to wack a lowbie mage, or kill some badly wounded warrior, etc.

As for weapon choice...since your posting on this forum, I'd go with sword Parries everything decently, and its easy to grab a decent sword at almost any rank. Could also carry a staff to use against APs and Thieves, since those classes don't learn staff. Though i'd suggest avoiding thieves altogether, since they tend to rock an assassins world if they are played correctly.

Other misc thoughts...

Dual wield two swords if you can, as opposed to sword+dagger. Daggers seem to be parried more often. Having some swords with attack types that bypass giant physical resistance is also a plus.

It's risky to wield anything over 15 pounds unless your packing +str gear to boot. Look out for some good light swords, I usually find that anything under 12 pounds seems to work well.

Dont throw other assassins, hand specs or dagger specs(with concealed).

Make sure you throw in some +dex into your gear. Some +hit is also nice, a hitroll of 20-->25 (at say around rank 30 or so) seems to work well. Then maybe a little +str if you like wielding heavy weapons, and just dump the rest into sspell and +hp.

Rom.