Wage wise, get to know what you are entitled to by law so you don't end up being fobbed off with a crappy wage. And don't immediately go for cash-in-hand if you are to be paying taxes or looking for steady employment - a company that wants a permanent employee but won't put it on the books may well not be on the level. Casual labour is different as they should sort that out with their accountant (such as my job) but if you are eligible to pay any sort of taxation on money you earn the onus will be on you to declare it or risk getting caught.
If you get a job with a written contract, read all the terms before signing and again be aware of your rights. It is illegal, for example, for an employer to just go ahead and dock your wages. If you work cash-in-hand you don't have a leg to stand on, so be wary of sudden alterations in your job description - although a verbal contract still carries some weight, it is very hard to prove your case.
It is also not compulsory for an employer to pay you for overtime or pay extra for bank holiday/national holiday work. And any job you accept, get to know ALL your rights as an employee - my first real fulltime job was at a busy hotel and they messed me around so much I left, but only later did I realise they were breaking the law by doing what they did as I had a contract. A bit of research and effort will go a long way...
(the above legalities apply to UK law - don't know about other countries)