How do you get a secret base also i beat the E4 what do you do after that?

You know where that old granny's house is? Just past the desert and between mauville and fallarbor? Well near there there will be a kid standing near a big green tree, talk to him, hell chat to you about secret bases, and give you a TM called "secret power" this can be used in battle as an offensive move, but it will also allow you to build a secret base. You can build secret bases in any of those large trees (theyre everywhere) or large clumps of grass (the bushes you see everywhere, too) or in those little square indents in rocky walls you see about the place. As it's a TM, you can use it only once, but after you get this move, one of the people down in the market in slateport offers it for sale whenever you want for under $5,000. As for what to do after beating the elite 4, you can hunt for Groudon, Latios and Rayquaza (theyre cool) and You can go hunting for the three regis (regice, regirock and registeel) but if you are going to look for those three, I suggest you get yourself a wailord and a relicanth and sail west of pacifidlog... Search for a small spot where you can dive... Have fun!

To get a secret base, get the proper TM for it. You get it from a guy standing facing a tree just north of the desert. (it might be tm 43) teach it to a Pokemon and then use it on the proper tree/bush/hole to make a secret base.

After you beat the elite 4 you can traine your Pokemon and go to the battle tower. You can slo ride the ferry.