In WvW there are mainly two types of approach you can take, roaming or joining the crowd. Roaming requires you to be alone or in a small group. Joining the crowd has you fight alongside with dozens of players. Both of these approaches more or less fight with similar scaled number of enemies.
Tank is a broad term. Are you going to be a power build or condition build? Do you favor AoE or single target approach? Do you like minions? As you can see the “tank” part comes mostly from gear and you are free to trait according to your needs. My suggestion is building in wells if you join the crowd as AoE is more important than ever in a big scale fight. Condition build doesn’t do well in that scale as cleanses fly across enemies quickly. What condition build can excel at is roaming as you need to focus your enemies more in that scale. For gear pick what you like: there are Dire, Soldier, Rabid, Knight, Carrion, Apothocary and Clerics with which you can be both “tanky” and do respectable to high damage. For well and/or power builds I would suggest dagger/dagger or axe/focus with staff as weapons. For condition and/or hybrid builds I would suggest staff or axe/focus and scepter/dagger as weapons. Rune and sigil options are broad and you really have to like one to go with. I suggest you check each one out in gw2skills or PvP map to find what you like. As for consumables, I suggest you to take either a condition duration increase food or life steal, depending on your build.
PvE revolves around mostly one thing, damage. You bring the best support by having the best damage gear, Berserkers. That is not to say you have to have the max dps build, however. For support, you can build on wells to both support your enemies and keep you alive. You have wide array of weapon choices, but I advice you to go with dagger/warhorn and dagger/focus for good damage and survivability, while you can slot staff for utility with the condition of spending time in staff as short as possible during combat. Your runes should be Scholar for max damage, while your sigils can be Sigil of Life and Sigil of Leeching, or anything else to suit your needs. For consumable you can use life steal food and dungeon/mob type specific potions.
