What are you after all ? A road bike (drop handlebars) hybrid (sturdier but for road/light trail use) or a mountain bike? Most bikes go small enough for 5'3" will ask dp which frame size would be best. The basic rule of thumb is the lighter you want, the more expensive the bike. Alloy bikes are great if you're not going to use it all the time for miles and miles, but if you want a road bike (for fitness or to do triathlons on say), a full decent carbon bike is at least a grand. Stay away from halfords, their lightweight bikes are just not, and they keep the bikes cheap by putting cheap parts on them that fail very quickly = more money in constant repairs. Their kids bikes are ok, we can't compete there, but adult bikes need to be fit for purpose. Oh God, I'm sounding like dp!
Go to a local independent bike shop, but have a good idea what you want, what you want to use it for, and a clear budget. Do some google first, so you don't get sold something you don't want! We see it so often, when someone gets such a bargain, it was the last one blah de blah; the bike usually doesn't fit and seemed a bargain as the bike shop wanted rid of it. We can get the right size bike in overnight, any shop can, but they always like to get shot of their stock bikes (as they have paid for them already). We are predominantly a repair shop, with just four shop 'test' bikes, and order in for each customer who wants a new bike.