After 40 years, rubber bands and garden twine would be an improvement! The main difference, as ludwig pointed out, is whether it is steel strung or nylon strung. And it does make a difference, think of them as different instruments as they are strung differently. If you haven't taken the strings off in such a long time it may be a good idea to give your guitar a bit of a clean. I would also suggest that if it is a reasonably good instrument you might want to take it to a guitar shop to be 'set up'. Basically a service but they will make adjustments to the action, cleaning the frets etc. This should cost about �20-�30 but is a sound investment as it will make your instrument easier to play and sound better too. The better it sounds, the more you will want to play and the more you play the better you will be.
For the record, I like to use 'Elixir' strings which are great and I just use a medium guage. I don't even know what size.
Good luck, whatever you do will be an improvement. Oh and one more tip, change your strings regularly : )