get all records from db from a specific datetime

You need to end your query with a select clause – and I think your rent period start time comparison is the wrong way round, unless I’ve misunderstood you – but I would also convert your whole query into the query syntax rather than the mix you have, e.g.

var parkingspot = from Address in _parkShareDbContext.Addresses
                  join ParkingSpot in _parkShareDbContext.ParkingSpots on Address.Id equals ParkingSpot.Address.Id
                  join RentPeriod in _parkShareDbContext.RentPeriods on ParkingSpot.Id equals RentPeriod.ParkingSpotId
                  where ParkingSpot.Active 
                  && (size == null || ParkingSpot.Spotsize == size)
                  && RentPeriod.Type == periodType
                  && !RentPeriod.IsRentedOut
                  && RentPeriod.RentPeriodStart >= realStartTime
                  select RentPeriod;

EDIT

However, as per the comments above, with complex queries like this one, you are usually better off writing a parameterised SQL query, or using a stored procedure to fetch data.

I’m also not sure why you need Address in there?