Well, that's easy.
Check your Slim model is compatible with CFW below, and if not, then your option is HEN (one click hack).
https://gbatemp.net/threads/hackable-ps3-models-and-information.538514/
The PS3 Toolset via the Xploit site will indicate if your PS3 can install CFW or not, anyway.
My preferred FW is 4.88 and I'm using its CFW. Just got no reason to jump to 4.89.