It is a bit easier, but most importantly usually cheaper, to buy a round-trip ticket rather than the two legs separately.
There are many sites available that will allow you to shop around for the best prices and schedules, though in the end it's often a better choice to book directly with the airline.
As there are no direct flights from the US to Freetown, you'll have at least one connection. If you're a first time traveller, it's probably a good idea to avoid additional connections, but less direct routes tend to be cheaper, so it's really your choice.
As an indication, airlines flying from Europe to Freetown include Air France, Brussels Airlines, KLM and Turkish Airlines.
Since you have at least two flights in each direction, it's probably a good idea to book the flights on the same airline or on airlines part of the same alliance.
Remember to check:
- the airline change and cancellation policies, if that's relevant for you. At the very least, you need to be aware of them. Cheaper tickets are usually non-modifiable and non-refundable, or only with significant penalties.
take into account time zones and date changes. If you see a flight arriving at
10:00+1 for instance, that means 10:00 the following day.
avoid short connections, especially if the airport will require you to go through immigration, pick-up and re-check your bags. Usually not the case in Europe if you stay within the same airport.
more generally, make sure you don't book a connection with flights landing and departing from different airports in the same city (shouldn't be an issue if you fly through Paris (CDG), Amsterdam, Brussels or Istanbul).
check for any passport/visa requirements, both at your destination and in transit cities (shouldn't be an issue for EU airports, not so sure for Istanbul).
Remember to sort out any passport/visa requirements before booking your flight!