There are a bunch of things that can be going on here.
The core problem here is most likely that too much of your body weight is being supported by your arms. Most of the time if you are riding in an efficient and upright position you want most of your body weight on the bike to be supported by your legs, and most of the remainder to be supported by the saddle. A tiny bit left over can be supported by your arms, but ideally that should be almost negligible. If you can develop better core strength and balance you can mitigate this problem.
The other thing you can do is consider adjusting your saddle. Having your saddle adjusted too high or too low may force you to support more of your body weight on your arms. Also, if your saddle is tilted forward that may slide your whole carcass forward and force your arms to carry too much of the load.
If you are getting a sore butt the first thing you should try is raising the saddle height, which will force your legs to support more of your body weight and relieve the pressure on your ass.
Note that I refer to a "saddle" rather than a "seat" because you in no circumstances want to support all of your weight on your behind. You are not "sitting" on your bike, you are "riding".
My observation is that the vast majority of cyclists, especially inexperienced cyclists, have their saddle adjusted too low.
A lot of bike shops, some physical therapists, and even some chiropractors can help you with bike fit. Before you go buying different grips or saddles it is wise to consult with a bike fit specialist. Honestly most saddles and most grips work fine for most people in most circumstances.