Being a Assistant (or Off-Duty, or Retired) you have no access to any part of the station excluding Maintenance, but it's good to play if you are a new player. They don't have any special responsibilities and there can be an unlimited number of them on the station. Assistants also make ideal applicants for genetic test subjects and other roles as they're not tied to any department and are therefore more or less expendable.
Assistant is also a good choice if you're simply interested in role-playing - you don't have any work to do so you're free to roam around and interact with the inhabitants of the station.
If this is your first time playing it might be worth looking at a few Guides and learning about the various jobs you can do.
As an Assistant, you can assist with various jobs around the station. Feel free to ask over the radio if anyone would like any assistance. You only have Maintenance access, so it'll be pretty hard to get anywhere, but this is still a good job for new players. In order to get another occupation, one must head to the Head of Personnel's office. If you don't feel like contributing anything whatsoever to the shift, get yourself a drink from the Bartender and play some Nano-Mob Hunter Go on your PDA.
You can choose between either inventing a custom job title and having the Head of Personnel give you the title and appropriate access, and asking people over the radio if they need assistance. If you take the first option, consider a reasonable job that isn't something overpowered or self-antagging, here are some examples..
Actor, Animal Breeder/Caretaker, Boxer, Carpenter, Comedian, etc
Assassin, Firestarter, Gunman, Hitman, Killer, Serial Killer, etc
Aside from approaching the HOP for access. Cargo, Engineering, Medbay, and RnD all have Guest Pass Terminals that can print 30-minute long passes with their respective accesses. If you are interested in any specific department and play your cards right, you may just become an appreciated crewmember among your newly acquainted coworkers. The HOP has a Guest Pass Terminal as well if you only want temporary access or what other reasons.
Being a traitor Assistant, you're one of the least respected jobs on the station as many people tend to ignore them since they have the least amount of responsibilities to do. And being the lowest ranking personnel, you've been granted the easiest objectives available.
Take a look at the Primary Tools Storage and see if you can get anything useful, craft weapons, get a toolbelt and ready up. In addition, you'll also get the Pickpocket's Gloves in your uplink, which is exclusive to your role.
Changeling Assistants also have the same situation as traitor Assistants, likewise, they have the least responsibilities, easier objectives and etc. So stealth can also be an option as well. However, having the least access of the station, you might have a harder time to assassinating your target. Ideally you should try and impersonate someone who has better access, after absorbing them and disposing of the body of course!
The same goes with Vampire Assistant, once again, visit the Primary Tools Storage, craft a stunprod, be stealthy, and you're good to go on sucking blood from your targets! Though it'd be a little harder without your contraband items or your changeling powers.
The number one advantage of Assistant antagonist is your opponent's arrogance. Security and other crew members will typically assume you are inexperienced or stupid, so surprise them with your knowledge of how to wrench their disabler out of their hand and shoot them with it.
Never forget what is perhaps the biggest advantage of your "job" - the Head of Personnel will be happy to help you find a new profession, giving you access to that job's advantages, with no suspicion at all.