First step: Inform yourself about what you want to buy. There are a lot of coins around:
https://coinmarketcap.com/
Bitcoin/Ethereum/Litecoin can be bought on coinbase
Use this link https://www.coinbase.com/join/58d66553cd453c7bf1153bd8 to get 10$ in Bitcoin after your first purchase of 100$ - make sure that you use your own link for your friends - you will find it in your account by clicking on your name and "invite friends"
Other coins can be bought on exchanges like:
- Poloniex (https://poloniex.com)
- Bitfenex (https://www.bitfinex.com)
If you dont find your coin here, check out coinmarketcap for this information. For example Exchanges for IOTA:
https://coinmarketcap.com/currencies/iota/#markets
--> Important: To buy coins on an exchange you normaly need Bitcoins (you can buy them on coinbase)
If you have all the accounts (coinbase and if needed poloniex/bitfinex etc) make sure that you buy a hardwarewallet, which can hold you coin.
U DONT WANT TO LET THE COINS ON COINBASE OR AN EXCHANGE! (because when the plattform is down, your coins are lost - that happened to many people)
Here you can find hardware wallets:
https://www.ledgerwallet.com/
https://shop.trezor.io/
When you have your hardware wallet you can start buying coins and transfer them into your save hardwarewallet.
This is a really short explenation - feel free to ask questions :)
have a nice day and good luck with your crypto.