1st - always start with a weapon you are comfortable with. You COULD try "meta" weapons for each mode, but if you are unsure/uncomfortable with the weapon you will not be successful. So, pick a weapon that you are good with already and play to its strengths.
2nd - If you want to use a meta weapon, blasters are ridiculous because they can take out people on the tower easily. The Lunas and Rapid Blasters are especially good. The splat roller and krak-on are good because you can stalk the tower and take out people hiding on it. The splat offers the suction bomb, which is solid for making squids get off the tower, and the killer wail which can dislodge people as well. The krak-on gives you beacons, which you can put on the tower to help people jump or absorb hits to help you push, and the kraken which is really good at loafing around the tower (best on maps without water). Personally, I like the .52 gal and custom dual squelcher. The gal has great splatting power, a wall, and the wail (which I find really useful in TC). The CDS is great at support and map control, beacons if you have to get on the tower, and the wail. I'd stay away from the short range shooters unless you are already awesome with some.
3rd - if you aren't good at riding the tower then don't. It is really important to splat the enemies, so you need at least two people off the tower, taking people out. I love this role as oftentimes enemies are focused on the tower and are easy to splat. Keep an eye on the squids up top: if they have 0-1 squids up, jump on the tower ASAP. More people on the tower the faster it goes. BUT Don't push if you don't have to - protect your lead. A small lead being defended is better than an unneeded push that ends in a wipe.
Good luck!