You won't be tapping directly into the electric feed, unless you have a really unique setup.
First off, make sure you have the electrical capacity to add a spa- if your existing panel is full, it is possible that you may be at capacity. You'd need an electrician to estimate that for you. That could equal a service upgrade ($$$). If you do have existing capacity, but your panel is too full to add the double pole breaker, there are a couple options. One is to replace a couple existing breakers with the thin breakers, where you can put two of those in the space of one standard size breaker. This is possible for many panels, but you need to be sure you get the breakers that are approved for that panel. If that's not an option, you'll need to have a sub panel installed. You'll still need to put a two pole breaker in your main panel to tap the power to the sub, so likely you'll still need to move existing circuits to the sub panel. Unless you are experienced at DIY electrical work of this nature, get an electrician or two in to advise you and offer bids for the job.
Brewman