Blackwall is going to be tank in your party, so give him the best armor and shield you can find. He'll be responsible for keeping aggro, so get the enitre left side of the Vanguard tree (War Cry, Untouchable Defense, It'll cost you, Unbowed and upgrades). Next, go for the Sword & Shield tree and grab Shield Wall and Payback Strike. Any other abilites from this tree are optionial, but you might consider investing a few more points here for the passives. When Blackwall gets his specialization (Champion), go for everything in that tree as Champion is the best tank specialization. As for tactics, set any abilites that generate aggro and guard (War Cry, Unbowed, Shield Wall, Line in the Sand, Walking Fortress) to preferred.
Solas is the support mage, who's main job is keeping the party alive and debilitating the enemies. In Spirit tree, grab Barrier, Guardian Spirit, Peacful Aura, Dispel, Rejuvenating Barrier and the upgrades, of course. You can get Mind Blast and Revive too, but if your tank does his job right, you won't need it and there are more useful spells in other trees. Solas should have Winter's Grasp by default, it is a useful offensive spell and a good starting point for a few more talents in the Winter tree. Get the upgrade for Winter's Grasp, then go for Mana Surge and Winter Stillness as they help you cast spells more often. You can go for Fade Step if you like, it can be useful avoiding enemies should they come after your mage. And as for debilitating enemies, look for the Rift Mage tree, Solas' specialization. Get everything from here including the upgrades, and Solas will become quite efficient in weakening the enemy. As for tactics, set Barrier to preferred and in combat you might want to switch often to Solas and use some abilites (especially AOE spells) manually.
Sera will fill te role of the DPS rogue. You'll want to focus on two trees: Archery (go for everything, including upgrades) and Sera's specialization, Tempest (again, everything, except the upgrade for Frost Flask, as it taunts enemies and we don't want that). You can also get Stealth from the Subterfuge tree as it can help if your archer is attacked, but I found myself doing okay without it.
And finally, your chacracter, who is another DPS rogue. You need to focus on two trees: Dual Daggers and Shadow. Dual Daggers have great offensive abilites, and one defensive (Parry), which can be the difference between life and death to rogues as it lets you counter the enemy attacks. The abilites in Shadow let you stay undetected (Easy to miss is great as enemies won't notice you if you're flanking them, so you can keep poking their backsides without worrying about being attacked). As for specialization, I'd say go with Tempest or Assassin, but I have to admit that aside from managing Cole, I don't have much experience with dual wield rogues.
Other than these, make sure everyone's equipment is up to date, they're wearing the right trinkets and set their behavior in the tactics screen to follow themselves, so they won't get into too much trouble trying to "help" another party member.