Storj Labs cannot make specific recommendations of which exchange you should use, but you can find the exchanges that trade the STORJ token listed here or here. Please note that gas prices for sending your STORJ tokens to an exchange may vary greatly, so please be sure to check the current rates on Ethgasstation before proceeding. It may be better to wait for gas prices to drop before sending your transaction.
Direct trading for USD or Euro is possible now on the Kraken Exchange (this is not an endorsement of Kraken, it is up to every user to make their own assessment which is the best exchange to trade on for their own circumstances). Kraken also offers other fiat currencies for trading with Bitcoin and/or ETH, so it may be possible for users that want to trade for one of the other listed currencies to do the entire swapping process from STORJ to fiat on the same exchange with Kraken. Coinbase and Coinbase Pro now also have added STORJ-USD and STORJ-BTC trading pairs (Coinbase also allows direct conversion of STORJ to any other crypto asset listed on the exchange). For countries that don't use USD, Euro, or one of the other currencies available on Kraken or Coinbase as fiat gateway, you would have to first convert STORJ to either Bitcoin or Ether on any of the exchanges that has STORJ-BTC or STORJ-ETH pairs, and then trade the Bitcoin or ETH for your local currency at a cryptocurrency exchange that operates in your country. The Binance-US exchange now offers direct trading of STORJ-USD as well as STORJ-USDT pairs, while the Binance-International exchange currently supports BTC, ETH, USDT and BUSD trading but not direct fiat pair. Gemini ActiveTrader now also supports trading of STORJ in select countries, and in the US this includes the state of New York.
Be aware that centralized exchanges will most likely require you to first pass KYC requirements before you can start trading. There are also some swapping services available like https://coinswitch.co/, https://simpleswap.io/ and https://changelly.com/ and many other similar ones which let you just specify your sending STORJ address and receiving BTC or ETH address and make the swap almost instantly. This is more convenient than using an exchange, but it may involve a slightly higher fee (the price of the convenience).
You can try first with a small amount of STORJ if you are not sure how it works at first. In any case, please do not leave your tokens, BTC or ETH for extended periods of time on a centralized exchange that does not let you hold your private keys yourself. All exchanges can be targets of hacking attacks at any time, so best to just complete your trade quickly and withdraw your funds to your own wallet or bank account. Also, before you use a centralized exchange that works for your country, be sure to find out if they have any restrictions on how much or how fast you can withdraw your funds, if this requires KYC and what fees they will charge, otherwise you may have some unpleasant surprises later. Please use the exchange services' support forums for further help, we cannot give you support here for all the existing exchanges in the world, that is outside the scope of this project.