To buy a Bitcoin you need to find a local exchange (in the country where you live).
Step 1: Register with the exchange
Step 2: deposit money into the exchange (from your bank account)
Step 3: Purchase your Bitcoin on said exchange
When you say “cheapest”, i assume you mean the lowest denomination? Bitcoin is divisible to the 8th decimal place. part units of Bitcoin are called satoshis (named after the unknown person or persons who designed Bitcoin and created its original reference).
Most exchanges will have rule dictating the lowest number of Satoshi’s you can buy in a single transaction, so simply take this number and multiply by the prevailing price of the bitcoin on your local exchange:
You want to buy 0.01 BTC
Current price: $2473
therefore: 0.001*2473= $24.73
Bitcoin does not have any tangible asset to back it up (but then again, neither does Fiat). It’s value is determined by supply and demand and the sentiment that Bitcoin will become mainstream in the future.
Bitcoin supply is limited to 21 million and the scarcity element of this also makes it more valuable.