Buy a table saw or circular saw
Just my opinion: circular saw first (and build/buy yourself a guide fence for super-straight cuts); cordless jigsaw for rough parking lot work (can be a relatively low-end/cheap unit); tablesaw eventually.

There are cordless circular saws on the market now that are pretty darn powerful (look for anything 'brushless'), but they are stupid pricey and if you aren't using them regularly, you won't get good value out of an expensive battery that will die over time.