If you are already transmitting and receiving using WSPR, then you are already on track for using the FT8 mode; there is not much more to set up.
First of all, check your audio settings are optimal, which you probably have already done for WSPR. Set the digital gain (TX power) slider for each band you want to use. Check the ALC does not overdrive on your rig. Set your rig TX power to your desired level, probably max. 50W for FT8 if your rig is 100W full power as FT8 is a constant carrier mode. The WSJT-X manual is a good reference.
Try calling CQ on a clear frequency. 40m or 20m are probably the best bands to try first. Shift+click on the waterfall to set your TX freq.
Wait and see what happens. It works just like WSPR, except you're making an actual QSO and it isn't as sensitive as WSPR for DX. FT4 can be fun too. It is less sensitive by a few dB than FT8 but it is made up for by the fact is often less busy, which means it can sometimes be easier to work stations further away. JT65 worth a try, but it is not very popular now.
I have a pro-tip also, if you want:
I recommend operating with Split mode if you are using CAT control to your rig. Use the "Fake-it" option. Doing it this way makes a much cleaner TX signal and you won't have to adjust your audio settings when you change from a low audio frequency to a high one. WSJT-X will keep the audio mid range and your RF frequency will be altered to compensate, so the DX station will hear you at 250Hz for example, but your audio output doesn't have to go that low.
Hope you have fun with FT8