Parking is general very available at beaches like Orient, where there's lots of sandy spaces to park behind the beach bars. Big exception for parking is Dawn Beach, where the Westin built an ugly building and blocked all the free parking.
In Philipsburg plan to park in one of the paid lots, because they are very quick to tow cars on the street even if it appears the spots are legal. There's a lot at the at the end of the beach walk area (near the Greenhouse restaurant) that's convenient. There is some free parking by the salt pond, unless the construction's going on in that area, and it's about a 4 block walk to the shopping street. It's fine during the day but don't plan to park and walk any distance on Philipsburg at night though, for safety reasons.
In Marigot, there's parking near the "market" area and the Marina Royale area, both of which have shopping and restaurants.
Most regulars to the island tend to use the independent car rental places, for both price and convenience. If you search on travel talk online you'll find lots of discussions about favorites. Most cars are small, which is actually good on the island for getting around and parking.