A mortgage is a loan from a bank or building society specifically to help you buy a property.
In return for giving you the loan, the bank or building society charge you interest and take a security or charge over the property you are buying.
That loan is repaid over a number of years (usually 25 years or more) by monthly payments.
If you don’t keep up the monthly payments then your home will be at risk. The bank or building society is able to call up their security, have you removed from your home and then sell your home.
There are various types of mortgage but all will be on a capital and interest basis if you are a first time buyer. This means that your monthly mortgage payments will be made up of the interest and a small amount of capital repayment (ie part of the loan is repaid).
Most often first time buyers go for a fixed rate mortgage. This is a mortgage where the interest rate is fixed for the first few years (usually 2 or 5). The benefit to you is that the monthly payments are fixed for that period and so gives you a degree of certainty.
Usually, the more you put in as a deposit, the better the rate of interest you can get.