After The Interview
It's unlikely you will be offered the job at the interview itself, but if you are offered the job on the spot then it's fine to ask for a few days before making a decision.
Send a thank you letter or email
After the interview has finished it sometimes helps to write or email the interviewer saying how much you enjoyed it, confirming your interest in the job and offering any additional information that might help them in their decision, taking the opportunity to detail the key advantages you bring to the job. If you do this, it's best to do it straightaway because the interviewer will usually make a decision within a few days of the interview.
Always ask for feedback whether you get the job or not
Most importantly if you DON'T get the job, so that you know what you need to improve for the next interview.
If you are offered the job and there is something that you are unsure about in the contract, it is best to mention it at this stage. For example, if anything is unclear about pay, bonuses, holidays or other conditions then make sure this is explained fully to you before you start the job. Issues relating to terms and conditions should always be given to you in writing. Having everything in writing will prevent any problems developing later on.