Classes and certs
I will try to recommend the items that will give the most marginal utility per cert. Aside from the entry on Infiltrator SMG, the guns are all 325-650 certs.
Spread your certing around a little. As an example, it's much more effective to cert your LA Jetpack 3 times than it is to cert your LA Nanoweave armor once (assuming same cert cost.)
Avoid it unless you really like it. Infils are shit at farming until you can afford the Armistice (1000 certs + attachments). If for some god forsaken reason you like sniping, a good OHK headshot sniper is the MB774 if you don't like the semi-automatic default. Dump some points into the motion spotter and recon detection device. Learn to use them, they are your platoon's best friend. You can't go wrong with the default cloaking and Nanoweave for ability/armor for when you eventually go CQC with Armistice. Claymores also causes massive butthurt, but if you're long ranging it, its not necessary.
Light Assault
Default TRAC-5 is fine, it's controllable and accurate. Slap a 5 cert scope on it, and eventually the grip or the laser sight (since the laser sight nerf from eons ago I tend to favor the grip now.) Default pistol will do you just fine.
Put some points into your Nanoweave armor and default jetpack. They're really good and inexpensive up til the last level.
This class shares its gun pool with the Light Assault. It has one additional gun, but that's not important for now. You'll be using the TRAC-5 just like the LA, if you have attachments from the LA they will carry over and vise versa.
Tank mines are great. 2 will kill MBTs, 3 will kill sunderers provided they don't have mine guard.
If you pick an engineer, you're probably going to be in a vehicle or repairing maxes. Cert into your repair tool, Nanoweave or Flak armor and a few value points in Ammo dispenser. While on Engineers, do not use turrets unless it's a coordinated turret hold. They're good for stationary cover though. Do NOT cert your default turret.
Medics are pretty good for cert farming.
Save up and get the TAR. It is hands down the best close-medium gun for the medic. After certing your Nanoweave armor a few levels, prioritize your Revivr, put some points into your AoE heal ability. Consider revive grenades too, they bring in quite a few bonus certs in large fights.
Almost everyone should have a half-decently certed heavy. This is where you have to make a choice. The HA has 3 different shields, they will serve you differently depending on how skilled you are.
If you hang around 1.0 KD, the default one is good enough. It will really give you a boost with your first engagement (and subsequent ones if you live long enough), but this ability is kind of expensive to cert, and recharges quite slowly til the last two levels.
Resist shield
My personal favorite. This provides a little less effective HP than the other two, however it's pretty much available 100% of the time. Combined with the Advanced Shield Capacitor (NOT NANOWEAVE, THE RESISTANCES DO NOT STACK FOR THESE TWO) and medkit spam, there's a lot more daka and less waiting for shield. With medkit tanking, this makes you harder to kill than the other choices. Certing Resist shield is low priority. The resistance % is the same regardless of level, as is the charge rate. The downside is you're at a slight disadvantage in HA vs HA fights when your opponent has a full default/adrenaline.
Adrenaline is the third choice. IIRC it's the most expensive one, and doesn't shine til the last two levels.
Go with Advanced Shield Capacitor if you are using Resist shield, Nanoweave for the other two.
The Carv is fine, but I personally prefer the MSW-R. It's more accurate in every way, has faster reload at the cost of only 50 bullets per mag instead of 100.
The default dumb-fire rocket is fine, though you may want to consider the air-lock rocket for when we're getting bullied by ESFs. Also, the damage on the rockets are really fucking misleading. Despite the similar damage values on the lock-on and dumb-fire rockets, they are not the same when it comes to armor. For example, all dumbfires will OHK or put the ESF you hit on fire. The lockons will take 3 hits to put them on fire. The only damage they share is when you hit infantry/maxes.