Know thy weapons.

Three things.

Small-arms.
Anti-infantry.
Anti-tank.

Small-arms is a catch-all term for the basic guns an army gets. In this case, it refers to bolters, pistols, shotguns, and stormbolters. Compared to what the imperial guard gets (lasguns, weaker shotguns, pistols, autoguns) this is a very good deal, but compared to tau (pulse rifles, pulse carbines, burstcannons, kroot rifles, smart missile system) it looks much, much less potent and impressive. But really, is it as impotent and unimpressive as the internet sometimes tells you? Not quite. Unlike what you've been told, marine toy guns are dangerous.

Your small-arms mostly have an AP value of 5, good strength, decent rate of fire, and the units themselves more often than not come with high ballisticskill. These factors mean marine small-arms is a potent source of damage, but like all other small-arms, its primary use lies in killing grunts. When unleashed, chunks of light infantry dies.

It's true that 9 veteran guardsmen aren't worth that much in points, but in model real-estate, this is impressive, and destroying the final one earns you a killpoint - alternatively, gets rid of a scoring unit. Marine small-arms is a serious threat to light infantry, and your high strength, accuracy, and rate of fire make prolonged exposure to it a concern for heavy infantry as well, but most types of armor is pretty much immune to it.

Anti-infantry is exactly what it sounds like, and you get a lot of it. Several different flamer templates, blasts, multiple medium strength guns, and plasma are all at your disposal. Some anti-infantry, such as the autocannon, the assaultcannon, and all plasma, is powerful enough to be used as anti-tank in many circumstances.

Anti-tank is there to slay armor. Being space marines, most of your anti-tank comes from melta, but there are a few other sources. All melta, krak missiles, lascannons, demolisher cannons, and that's it. It seems like a small list, then you realize melta is very, very good, available to almost every unit, and even adds a measure of duality, since it destroys all kinds of infantry, too, in addition to every single vehicle in the game - bar one.

Marine combat, as has been touched upon already, is powerful, but output is low. Where orks and eldar get to throw forty or more dice, you only get around twenty with a full squad. Versus vehicles, this number is even lower. Units with powerfists, chainfists, krak grenades, meltabombs, dreadnought close combat weapons, thunderhammers, and even relicblades all function well versus armor. Chainfists are particularly effective, but they're fairly rare. The problem lies in the fact that vehicles tend to move around. Moving vehicles are hard to hit, and even harder to engage with slow infantry and walkers.

Because of all this, trying to destroy armor in combat should be a desperation move - not something you casually attempt. If a vehicle is close by, and hasn't moved, feel free to engage it with krak grenades. Enemy dreads coming your way? Might be better to move back a bit, rather than rely on the squad's powerfist and four krak grenades.

Always use the proper tools for the job at hand, while remembering that many of your guns are effective against both armor and infantry. Plasmaguns work against some vehicles, but meltaguns are better by several orders of magnitude, and cheaper.

If given the choice between slagging a transport with meltaguns, or hitting it with krak missiles from a unit farther back, always pick the meltaguns. Your units are powerful, but few. You need to get the most bang for your buck, so should use your firepower as efficiently as possible. Slag it with melta, then throw frag missiles at the passengers that pop out. Charge any survivors with the meltagunners.

Simple and effective, but forgotten by many people - new players, especially.

Not counting this one, only two 'n00b' parts remain, then it's on to the meat.