Shepard: probably a soldier, as you need someone to complement Tali's fragileness as she is mainly in support role in combat. If Shepard is the main tank and gunman in the group, he'll need to be durable and deal a lot of damage with guns.
For durability, level up Fitness to increase shield recharge and health.
For weapon damage, level up Combat Mastery to increase weapon damage. There is one upgrade (squadmate weapon damage vs headshot damage), I'd say take headshot damage as most of your party members will be using their abilities more anyway, but if you are not good with headshots, the squadmate weapon damage is worth considering. Whatever floats your boat
Frag grenade is a powerful ability, level it up at least to level 3, then it's your choice - go for damage or radius (I prefer radius), pick capacity upgrade, and choose shield overload only if you are not planning to take Energy Drain as a bonus power, as a soldier lacks effective anti-shield ablities.
Concussive shot is another really useful ability, as it can detonate combos primed by other characters. Be sure to pick its AOE upgrade, works wonders on canon fodder enemies.
Adrenaline rush is only worth considering if you are using mainly full automatic weapons or snipers. It has a nice upgrade that grants you briefly high damage resistance.
As for the ammo powers, use them as you see fit, it might be worth investing a few points in each type of ammo. Incinedary and Disruptor are nice as they provide anti-armor and shield cabalities, but other abilites do them better.
For equipment, I'd say take a full auto assault rifle (Avenger, Lancer, etc.), a heavy pistol like the Carnifex and a shotgun that deals good damage (Eviscerator).
Tali: she's going to be combat support, so build her accordingly.
Her unique ability, Quarian Engineer gives her better recharge times and stonger powers, be sure to choose those upgrades.
One of the best powers she has is Energy Drain, that can deplete enemy shields, detonate tech combos and refill her own shields in the process. When upgrading, choose upgrades that add radius.
Combat Drone can be a useful ally on the battlefield, as it can distract enemies, setting them up for a nice AOE attack, but it is rarely worth upgrading past level 3, as it is quicker to spawn a new drone than keep the old alive. Plus, it deals little damage.
Defense Drone is pretty useless, unless upgraded to level 6, where it can knock down enemies with a 30% chance that approach Tali, which might help keeping her alive.
Sabotage is only really useful during the Quarian-Geth story arc, but you might consider keeping it as it can help against Cerberus turrets. In that case, level 3 should be enough for everything.
As party members are not affected by weapon weight, give her the biggest, meanest heavy pistol you can find (Carnifex should be good) and a powerful shotgun that is not the big, powerful, single shot krogan shotgun which's name I've forgotten (the one Grunt used in ME2).
With this setup, Shepard can prime targets with incinedary ammo which Tali can detonate with energy drain.