The simple answer is you're trying to thread two different types of fittings together and that's why it leaks. I'm not exactly sure what the fitting on the left is, but I'd hazzard a guess at 1/8" iron pipe. The fitting on the right is a JIC/AN type of fitting. You need an adapter between the two. Wrap the fitting on the right (iron pipe) with teflon tape and get an adapter that goes from it to the correct AN size. The AN fitting will be a tapered end that does not require any teflon type as it's made for pressurized liquids.
You should be able to pick up the correct fitting at any store that can custom make hydraulic hoses and you MIGHT be able to find the correct fitting at some automotive parts stores that carry high performace hose ends like Earl's etc.
Rick