If its never been painted before, use a block sealer product that contains portland cement. Dry-lock contains portland cement, but is more expensive than other products. Dry-lock is the best product if the block leaks or you anticipate leaks. Once again, more infor would help! Is the garage below ground? At the base of a hill? Several factors determine how you can proceed.
My basement never had any signs of water, so I used a less expensive block sealer that contains portland cement. Use a sealer against unpainted block, then finish with regular paint.
If there is any signs of mildew, scrub the area with clorox first before sealing or painting.
In the final coat(s) of paint mix one or more packets of mildew inhibitor to slow/prevent the growth of mildew. Depends on where you live.
Was the garage painted before? Is it heated? Internal or external garage? Where you live? Above or below ground? These are some of the factors that help determine what is the best thing to do.