Depending on whether you're playing 6th or 5th gen will determine which Pokemon can be used in which tier. If you are looking to play a lower tier in 5th gen (B/W & B2/W2) then you actually have 3 to pick from, bar LC. In 6th gen (X/Y), however, there is only one tier lower than OU and that's UU Beta.
This is a list of your 5th gen tiers. If you have a strong familiararity with how tiers work then you shouldn't need an explanation on which Pokemon slot in where. if you don't however, just follow closely. The tiers are listed in order of the number of Pokemon with usage. Ubers is the first tier listed, you can use any Pokemon in it. However, once you reach OU, the Pokemon listed on the Ubers section are unallowed, with some notable additions (like Drizzle Politoed and a Pokemon with Swift Swim are not allowed on the same team). Then you reach BL, or Borderline, which isn't really too important until you reach UU, which bans BL, OU, and Ubers. After that you get into BL 2, which is just like BL, but this time pertains to RU, where anything BL 2+ is banned. Then you get into NU which is the lowest tier (LC doesn't correspond with the other tiers) and everything above it is banned.
Then there's UU beta, which is currently the lowest tier in the 6th gen that, like 5th gen UU, bans anything BL+. Pokemon banned in 6th gen UU Beta:
All forms of Arceus, Blazekin, Darkrai, Deoxys, Deoxys-A, Dialga, Giratina, Giratina-O, Groudon, Ho-Oh, Kyogre, Kyurem-W, Lugia, Mewtwo, Palkia, Rayquaza, Reshiram, Shaymin-S, Xerneas, Yveltal, Zekrom.
Aegislash, Alakazam, Azumarill, Blissey, Breloom, Charizard, Clefable, Cloyster, Conkeldurr, Donphan, Dragonite, Espeon, Excadrill, Ferrothorn, Forretress, Galvantula, Garchomp, Genesect, Gengar, Gliscor, Goodra, Greninja, Gyarados, Heatran, Infernape, Jirachi, Kangaskhan, Klefki, Landorus, Landorus-Therian, Latios, Lucario, Mamoswine, Mandibuzz, Mawile, Pinsir, Rotom-Wash, Sableye, Salamence, Scizor, Skarmory, Smeargle, Starmie, Sylveon, Talonflame, Tentacruel, Terrakion, Thundurus, Togekiss, Trevenant, Tyranitar, Venusaur, Volcarona, Dialga, Giratina, Giratina-O, Groudon, Ho-Oh, Kyogre, Kyurem-W, Lugia, Mewtwo, Palkia, Rayquaza, Reshiram, Shaymin-S, Xerneas, Yveltal, Zekrom.
Now, I know you use Pokemon Online, but there is a place with a lot more cleaner tiers called Pokemon Showdown, which is faster, has a better teambuilder, and its where most of use DB users hangout. Here's a link.
Now what you do is you select Pokemon among those guidelines, Pokemon Online would normally fit you into OU if you don't qualify, so make sure you leave off any Pokemon banned from the tier you wish to play in. If you don't you will end up unmatched against your opponent. My suggestion to building a team is to use a framework like:
Your sweeper is the Pokemon who you build your team around, with the Breaker being the Pokemon who break through the walls of your sweeper. Your Lead sets up hazards, your Pivot takes hits and normally can be seen with U-turn or Volt Switch with a good Pivot ability like Regenerator or Intimidate. Your speed is just a really fast Pokemon with either powerful priority or a Choice Scarf. For example, if you want to build a team around Mega Medicham, you'd want Breakers, stuff that hits Ghost who wall it, a lead to set up hazards so HJK gets the guarenteed kill, and speed to mop up whatever else.