Carolcapos

While doing as many sidequests as you're willing to put up with is a must in the vast majority of RPGs, I'd recommend you pay special attention to the ones that reward you with Department Store Stickers for the early part of the game. Managing the EVs you earn is going to be very important for the later part of the game, and since early game will always screw your spread over (not much you can do about that, unfortunately ), being able to remove unwanted ones with the Friendship Berries is going to be a major boon. The Power Items will also be helpful for building up the EVs you want, since they'll give an extra 8 of their respective stat (8 Speed for the Power Anklet, for example). Both item categories are only available through the Department Store, which is ultimately what keeps the place relevant for most of the game.

Another good idea is to keep a varied rotation of pokemon rather than just a team of six. This'll give you more flexibility against the tougher fights in the game, since being able to swap out a weak link for something more capable in the specific matchup (like, say, swapping a chesnaught out for a crobat against the Fairy Gym Leader) could very easily mean the difference between certain defeat and a hard-earned victory! It also helps you keep from going over the level cap so easily by spreading out the story-required Exp. gains a bit, though that does become a temporary non-issue between the Fighting and Flying Gyms (the level curve at that point is a bit wack).

Heya!

Is that your first run in Reborn? If so, I would largely recommend those three starters if you haven't made your choice already (Grass Type --> Snivy; Fire Type --> Torchic; Water Type --> Mudkip/Froakie). As for the mons you'll encounter in the wild, Poochyena is catchable as soon as you exit the Grand Hall and is pretty helpful for dealing with the first four Gym Leaders. Make sure it has Moxie though. Also, any Fighting Type Pokemon like Makuhita or Pancham will do the trick when you'll adventure in Obsidia Ward (Post Julia battle). Fletchling is also pretty underrated in my opinion, try to find one when you'll have access to this place: Beryl Cemetery. Tailwind setters can make heavy differences in Reborn, and I'm not talking about Flying Gems combined with Acrobatics. Well, at least, that's part of my strategies ^^ Otherwise, try your best to locate and pick up any egg you'll see, since it generally contains decent mons that can only be encountered later in the game (Bronzor, Ducklett, Nincada, Deerling, to mention a few). Another protip: Tyranitartube's nuzlocke made me realize how tanky Gulpin can be for early game content. Don't hesitate to check the trash dumps in Lower Peridot Ward, this is where the Poison guy is hiding! I think that's about it!

Glad to help, and good luck