new certificates from go daddy?

I recently fired up my old imac and have just been using it to watch YT videos and play games on, but now I've signed into Chrome and connected my gmail, etc. I'm past the update stage, but in order to upgrade, I'll have to add memory to the system. Fortunately for me, I have the only imac that allows the owner to do that. Unfortunately for me, I have been unemployed for 8 months and would prefer to wait until I have steady income, to make an a trivial expenditure like that. I am getting the big red triangle on certain websites, which I just bypass, as I am not entering sensitive info ...but, I am in the process of setting up an ecomm business, so I began to explore how to fix the lack of secured connection, and it's because of a number of expired certificates. How do I go about getting certificates that aren't expired that will work with my older (Yosemite!) system?