By law your employer must give you a written contract of employment.
This should lay out all your main terms and condition, such as rate of pay, place of work, hours of work, holiday pay, sick pay etc.
This is a contract and a contract is an agreement between two sides. It's not just them imposing stuff on you. Once you've both signed it they have as much obligation as you do to abide by the terms.
So you have to be paid on time, you have to be allowed to work the hours stated in the contract and you have to be paid for those hours.
If they don't stick by what you've signed for they are in breach, simple as that.
The other thing you might be issued with is a company handbook. This is also an important document because it will contain most of the company policies and procedures on issues such as grievance, discipline and appeal. There may be stuff in there about tips and service charge, maternity and paternity leave, pensions and other important matters.
Always keep hold of your contract, your handbook and any other documents issued by your employer.
If you come to the union for help on an issue we definitely going to want to see these.