Having an imp familiar attack with conventional weapons is usually a mistake…
While the imp familiar will possess its master's base attack bonus if that's better than its own, the rogue//warlock's base attack bonus is no great shakes. Further, while its outsider type grants the imp proficiency with simple and martial weapons, its Tiny size makes melee extraordinarily hazardous. Also, its Tiny size means any weapons the imp wields won't deal much damage. (A bastard sword designed for a Tiny creature deals only 1d6 points of damage, and the imp's Str 10.) Finally, because the imp takes no class levels and itself gains no Hit Dice, it gains no new abilities except those its master grants, and the imp is typically stuck with whatever feats it possessed when it was summoned. (This is all in addition to the imp familiar's fragile hp, of course.)
In short, in this long-time player's opinion, having an imp familiar engage in conventional melee combat—charging at a foe, weapon in hand, for example—just isn't a good idea. Something untoward will occur, and the imp will die. And, when it does, unless it's the subject of the 6th-level Clr spell revive outsider [conj] (Spell Compendium 175)—the casting of which is a service with a price of at least 5,660 gp if it's available at all—, it'll be a year and a day before the rogue//warlock can get a replacement (see here). This can mean a lot of sessions with two useless feats on that character sheet.
If an imp with a conventional weapon is an absolute, a Tiny composite longbow (Player's Handbook 117, 119) (50 gp; 0.75 lbs.) is probably the best choice. Combine its range increment of 110 ft. with the imp's ability to fly and go invisible, that's like paying 1 cp per turn for a 5% chance of 1d4 points of damage out of nowhere. That's okay, I guess.
…But give that imp a wand, and that imp can still contribute during combat
Nonetheless, rogue//warlock is a good combination for weaponizing a familiar using unconventional means. (It's a combination beaten only by, like, two classes that each have an expansive spell list or by just one and a meldshaper.) A familiar shares its master's skill ranks, a rogue//warlock is going to keep topped off his Use Magic Device skill, and an imp's Charisma score is 15. This makes it so that the typical imp familiar of a rogue//warlock can—assuming the master's specced for it, and he should be,— fairly readily employ the Use Magic Device skill to activate a wand (DC 20). Might I suggest handing that imp a wand of minor image or scorching ray?
Also, might I suggest the rogue//warlock acquire a wand of familiar pocket [trans] (SpC 88) (1st-level spell modified to a 2nd-level spell by the feat Extend Spell at caster level 12) (360 gp/charge; 0 lbs.)? (This is the deluxe 24-hour version; cheaper models available.) In sum, while the familiar pocket spell continues, if the familiar and master are touching, either the familiar or the master can take a free action to utter a command word to have the familiar enter a comfortable extradimensional space where it's totally safe (but the paranoid can see here). Similarly, either the familiar or the master can take a free action to utter a command word that causes the familiar to exit the pocket effect.
This means that the master could take a free action to have the familiar exit the pocket effect, have the familiar take its turn normally to, like, activate a wand or fire its bow, then either have himself or have the familiar take a free action to command the familiar back into the pocket effect. Really, I can't overstate how important the familiar pocket spell is to keeping a familiar safe in combat.
Note: I DMed a campaign that included a PC who weaponized his multiple familiars. (I've discussed Jalas here.) The campaign ran from levels 1 to 15, and Jalas was the only original left from the level 1 group at level 15. So, yeah, weaponizing familiars can work, but, to be sure, it's a risky and fragile strategy. Also, the Dungeon Master's Guide feat Improved Familiar (200) lists the imp familiar as available to a creature that possesses an arcane caster level of at least 7. A typical rogue 3//warlock 3 possesses an arcane caster level of 3.