I've built a lot over the years. But, it is so easy to make a mistake and waste a lot of money... In any case, at this point in time it seems cheaper and easier to get one ready-built. Most PC suppliers are struggling for business and they have ways to let you customise the components.
Just make sure you don't buy from a company that is about to go bust with your money.
In the UK, if you buy with a credit card (but NOT with a debit card), the credit card company is responsible for giving you your money back in that event. (Not sure if that applies for things purchased from overseas, though).