The amount and maintenance of margin accounts depends quite a bit on the exchange you are using and where you are located. If you are willing to invest larger amounts ($10,000+) Bitmex or Bitfinex would be good places to start. If you are looking to do smaller amounts, places like cex.io or Poloniex also offer margin trading if you qualify. Keep in mind that all of these have different rules and restrictions, so make sure you do your research to find the one that works for you.
Here is a resource for beginners as well on margin trading in the crypto space:
https://cryptopotato.com/bitcoin-altcoins-margin-trading-beginners/