What's essential in a team? Pokemon of course!
Joking aside, I agree with some of the points made here already, especially with Noir. The beauty about competitive battling is that there are so many ways to go about building your team that sometimes it's just overwhelming. Having some sort of core tactic/theme in mind helps focus your teambuilding. This may be a certain Pokemon or two that you want to use/show off. Once you have that in mind, think about their weaknesses. Where do they lack? What are their counters? Once you know these things, think about which Pokemon support your core in such a way that these counters can be dealt with.
For example, you may want to build a team around Quiver Dance Volcarona. What are its weaknesses? Well for one, Stealth Rocks so make sure you have a spinner on your team (or someone with Defog). You may also consider adding a Rain Dancer/Drizzle Pokemon on your team to abuse Hurricane on Volcarona. This of course reduces the effectiveness of any Fire type move your Volcarona could have.
On the topic of entry hazards, again you should pick them depending on what type of team you're trying to build. If you want to go for a stall team (why would you, those are SO BORING) then perhaps you may want Stealth Rocks, Spikes, and Toxic Spikes to make stalling easier. Because you'll be putting up so many hazards, your opponent will be looking to spin them away as soon as possible so you may want to put a ghost type on your team to block the Rapid Spin.
TL;DR: Pick a Pokemon or two that you really want on your team and build your team around that core.