New

I have recently had a mountain bike gifted to me by a friend. I've never owned one and I grew up thinking they were lame. Well, I drove it about a week ago and I love it. I haven't rode a bike in about 10 years, but I felt pretty comfortable riding it. I have an issue with the seat though. It's super hard and actually hurt me. It's a little rough to sit down because of it. So I have a saddle seat in my Amazon cart, but I'm looking for other things to maybe upgrade such as the grips and breaks. I just need a little direction. Where should I look for these things and what should I look for? I am on a budget which is why the bike was gifted, but I would like at least reliable quality. Below is the exact bike gifted to me.