Barcelona airport is sufficiently big enough that you can find yourself lost in it, ot nor arrive in time for your flight because it was too far away and you didn't plan ahead. For these reasons, this map will be very helpful for you. Here is some information you will want to take into account if you do not wish to miss your flight.
There are many care rental companies available at Barcelona airport, but there are always some that provide better experiences than others. One company that I would not particularly recommend is Europacar, this is based on personal experience. They never fully explained the process well enough and they would not accept a €100 deposit from me in place of €150 (I only had €100 on me and a debit card which they don't let you pay the deposit with). If you have your own experiences with car rental companies that you'd like to share, leave them here, in the mean time, here are 3 good car rental companies:
In cities like London, and New York, you can catch the metro and end up at the airport. Currently you cannot take the metro in Barcelona directly to the airport, though a Line is currently being built that serves this purpose, the line goes through much of the city, including Plaza Catalunya, and stops at each part of the airport (Terminal 1 and Terminal 2).
Whether you're looking to relax on the beach, or have a thrilling weekend at PortAventura theme park, travelling to Salou isn't that much of a big deal if you know what you're doing. Depending on whether you've made it to the city centre of Barcelona and you want to go from there, or if you're travelling to Salou directly from Barcelona Airport, your options will differ slightly.
When you stay in Barcelona, you're probably thinking about which hotel you should stay at. The problem is, you're asking the wrong question to start with. The first question you need to ask is "Which type of accommodation should I stay in?". Well, although in terms of the accommodation Barcelona has to offer, there's plenty of types to choose from, the two main ones are hotels and apartments.