You can buy coins on one of the many dedicated coin exchanges, including Coinbase, Kraken, Anycoindirect, Chex, Shapeshift, LocalBitcoins, Bitrush, Bigstamp, Gemini, OkCoin and Coinmama. You can have a look on those sites to see which one offers the best price, but they'll likely be quite similar (otherwise you could make money by shifting from one exchange to another).

i think this is the best time to buy the coins because the value of the crypto currency has dicreased but in future the value will deffinetly get increases

You can buy it at some trusted site like sp564 answer, but I want to add little of my experience, because you say for cheapest price, I think the market price isn't different so far, the price almost the same, I sugest you to buy when the market is red, you can get the cheaper price, but you should learn more about how to analyze the market, you can learn it by Google, Youtube and etc and read some news about cryptocurrency. And if you want to get cheaper price, this is second way, you can buy coin ICO, the price is cheaper, but you should learn more about the project first to analyze how that project in future, thank you

go to the stock exchange, any, and there is a list of coins!

It also depends on your location. What is your country? Coinbase is the most popular internationally, I guess, if you're talking about buying BTC using fiat money. You can google the available exchange in your country. If you are Asian, some countries in Asia has so many local exchange that you can use.

But if you already have BTC and just looking for the right exchange to buy or trade with the cheapest price, I recommend Bit-Z. It's not as cheap as the cheapest but they have lower fees. . Trust me, Fees are eating your money.

Are you trying to purchase Bitcoins? If you want to purchase Bitcoins then you would have to see what exchange has the lowest price on Coinmarketcap.

well, first of all, are we speaking about bitcoin coins or other coins as well?
i know there is not much difference in fact, cause the actions you need to do in order to complete purchase are the same
so just choose where you want to buy coins - there are several exchanges and services were given to you already in previous posts, just check where lowest fiat deposit  fee,  and but at right price

Coins? You meant the alts and bitcoin itself? Well by that you should go and buy at coinbase if you are to look for bitcoins . And if you are looking into altcoins, better find a way for you to transfer your btc from coinbase to exchange.

There are lot of platforms available on internet from where you can buy the coins. You can also check the prices of the coins on various exchanges at one place i.e. on coinmarketcap.

Yes there a lot of coins to choose on buying coins. Coinmarketcap is the best guide for you to choose the best coins to invest. Op should read on what coins to invest that may serves as short or long term investment.