Picking the right starting weapon to cater to your playstyle is crucial to doing well in Halo 4. Each of the three base primary guns have very distinct statistics that can dramatically alter how you play, same goes for the pistols. We're going to ignore some of the later unlocks for now, but when you do finally get access to them experiment and see if any one of them suits you better.
The Assault Rifle is by far the easiest of the weapons to use with it's rapid fire rate and spray-and-pray philosophy. Using the Designated Marksman Rifle (DMR) requires a lot more accuracy, but it pays off in terms of distance, acting like a low-powered sniper rifle for those with steady hands. Between the two you'll find the Battle Rifle which doesn't really excel at anything, but is probably the best all-around weapon. Understanding whether you like to handle things up close and personal, pick enemies off from afar, or something inbetween is crucial to selecting the right weapon.
For secondaries, it's hard to go wrong with the Magnum and if you never unlock another, you'll likely be fine. However, careful use of the Plasma Pistol can reduce shields to nothing before you switch to your primary to finish an easy kill. The Boltshot is closer to a shotgun, but gives its user the choice between firing five of its ten chambered bullets or just one, making it rather versatile.