All you are doing by becoming a vegetarian is cutting out meat, poultry, and fish from your diet. This leaves most food items to choose from.
Cereals, bread, rice, pasta, lentils, beans, vegetables, fruit, nuts, cheese, milk, yoghurt, free-range eggs, biscuits, chocolate, and much more are available to eat. You don’t have to use any expensive “health food” shops; carry on shopping in supermarkets as before. You can make delicious meals such as lasagne, vegetable casserole, spicy bean burgers, egg and chips, pizza, cauliflower cheese, stir fries, a range of curries, soups and stews, nut roast (a Christmas special), or simpler snacks such as baked beans on toast.
Make sure that you buy vegetarian cheese; this has an alternative to animal rennet which comes from a calf’s stomach. All supermarkets stock a good choice; it will be labelled “suitable for vegetarians”. And carefully check the list of ingredients of things like biscuits to make sure that they don’t contain something like animal oils.