The "insufficient security level" is unfortunately to make it more difficult for those who we have to keep away from returning, there's no easy way other than to just hit the improve level button and wait it out,
Once you're on, you'll find a plethora of channels to choose from, if you scroll down a tad, you'll find a list of public rooms for various different areas of the UK, anybody is welcome in these channels. If you find yourself wanting to join a gang, you may find yourself using the gang channels below the public area, "official" RPUK gangs can have their own channels created here.
Below the gang channels you'll find the faction channels, NHS, then police, then poseidon. All of these channels are inaccessible unless you're a part of that specific faction.
Ultimately, whilst the teamspeak is huge (currently 653 channels), it's all spaced out and labelled so you shouldn't find it too hard to find your way around. And if you are stuck? Well, there's a channel right at the top "Join for Support" -- Does exactly what it says on the tin, double click and join that channel and await a member of the staff team, we'd be happy to help.
We also have a discord! discord.gg/roleplay -- We don't make much use of the voice-chat capabilities that Discord offers though you will find plenty of text channels.
Hope that's helped a little bit, let me know if you need any further help!
Enjoy the Altis Life.
//Gordon