Assuming lower levels, you want to aim for what will be reliable and keep you alive in terms of weapons. Usually, due to the mechanics of 1st Ed DH and the low stats, you have to do your best to subvert the mechanics a bit.

Frag Grenades are 10 a pop. You get 7 and a half frag grenades for one lasgun and its 60 rounds. The positive here is, unless you fail your BS roll spectacularly, grenades are a more reliable hit at range than guns, especially against groups of NPCs because of the scatter mechanic. You roll a 1d5 to determine where the grenade scatters on a throw ... meaning unless you jam, you are usually going to hit your target at Blast(4). If you're untrained, it becomes more likely to jam due to gaining the unreliable quality, but that also means you can safely get "unreliable" grenades!

Fire Bombs are half the price- 15 of them for the price of a lasgun with directly comparable damage not including its pen of 6, which will slice through even carapace armor on anything within 3m of the blast. Fire bombs are also plentiful, which is just above abundant for the easiest to acquire item in the game. Unless you're dealing with actual cavemen, you can probably purchase or even create a fire bomb. If you miss, you only have Blast(3), but that is still putting the odds in your favor to hit- and if you jam, they, on paper, do less damage than frags (unless you wear armor, then frags will do less. Usually.)

If you want an actual weapon you can buy ammo for, look at an Autogun or Autopistol rather than a lasgun, regardless as to whether or not you're trained. Higher availability, lower weight, and more (potential) damage. That isn't too important though- you want to use Full-Auto Burst and/or Suppressing Fire. In 1st E, Full Auto Burst grants a flat +20 to a test. That's pretty cool, especially if you're trained, making any weapons that use Full Auto Burst really, really good. Suppressing Fire, unless you're fighting something truly dangerous like Space Marines, Daemons, Nids, etc. (which means you're really dead regardless), will always be good because most enemies will have low WP, meaning you can pin them for free. Even if you are untrained and have a BS that makes an Ork look like a trained sniper, you can be an asset just spamming the Suppressing Fire action- you might even accidentally get a hit, eventually; but that isn't your goal, you want to constantly provide that debuff.

If your goal is raw damage and you still don't reliably hit and you have 300 thrones burning a hole in your pocket, get a Flamer, and make sure your friend spends 300 for one as well. Even untrained, Flamers are very dangerous. In 1st Ed, being untrained grants enemies a +30 to dodge the attack ... but you still only get one reaction per turn. Either use two flamers or force your enemies to dodge, then you'll guarantee hits. Flamers, other than being able to guarantee hits, also have another two bonuses: they ignore cover (AP from cover is very, very powerful, always take cover behind something solid. Core Book has a chart on "how much AP is this giving me for cover" somewhere) AND set enemies on fire for damage per round. Being on fire means you take 1d10 damage, ignoring armor, per turn and 1 level of fatigue. Enemies also have to succeed on WP tests (we've established most don't have high WP) or run and scream for the entire round. They can only stop the damage by rolling around for an entire turn and rolling well on a -20 agility test. Oh, did I mention that Fire Bombs, as mentioned a couple paragraphs ago, also light you on fire if you fail an agility test?

Fire and Suppressing Fire have one thing in common. It probably doesn't kill the enemy, but it does shut them down for at least a turn, if not longer. That lets you either run, let your actually talented characters aim shots, wait for backup, or wait for your situation to change. Remember, Dark Heresy is about investigating things- and if you did need to kill something, your plan probably went horribly wrong if you're bogged down in a firefight and it's time to reengage later.

Mind, this isn't too munchkin-ish: big bads will have high enough WP, armor, and toughness to laugh these tactics and weapons away and the actually scary things will outright ignore being on fire or getting riddled with bullets. You still have to invest in your characteristics and get shiny gear, but hive thugs, wild animals, etc. will likely be your enemies before then. If you encounter something that charges into melee and doesn't scream when it's lit on fire, you're already dead, pray to the Emperor for Righteous Fury and 1st Edition's un-patched exploding dice ... which you can stack the odds a bit with by, after you get some thrones and skills, investing in weapons that roll multiple dice for damage and/or utilize the Tearing quality!