It's not easy to help if you don't give us the right info...
You posted saying you were leaving Marrakech, and now it's Ouarzazate...🤪
Few tourists dare to take the buses, but they're actually pretty reliable, especially for the city center.
https://www.alsa.ma/marrakech/lignes
Personally, I use the bus in Marrakech without any issues... And if you can't figure out the bus numbers, just ask a driver at a stop which number goes where you want to go...
Near Jemaa el-Fna, almost all of them arrive and depart from there.
The main sites are super easy to reach by bus. The price is negligible...
Another option: in front of Jemaa el-Fna, there's a KFC, and just behind it, there's a paid parking lot—it costs between 50 and 70 MAD for the day...
There's another one near Riad Zitoune, next to Place des Ferblantiers... You can also park near the bus station and take a bus...
But driving in Marrakech can be intimidating if you're not used to it—it can be intense. The passenger will want to brake as much as the driver...
Another possibility: you can park at the Marjane parking lot on the Casa or Safi road... There's a bus that goes to Jemaa el-Fna...
That said, I wouldn't hesitate to book a riad in the medina for a first visit—it's so much nicer... Though you'll still have the hassle of taxis or buses to get to places like Majorelle (which, by the way, is near the bus station—don't forget to book online for Majorelle) and parking your rental...