Feel uncomfortable of resignation

I would like to resign in these two weeks since I have already accepted another offer. This is my first company and I am the only person to do the work. Though I know my current offer is better for me and current company cannot offer me the same opportunity, I still feel uneasy and some guilty to resign and confront my boss and colleagues. Honestly, most of them are nice to me. I don’t know how to bring the resignation topic. Just walk into my boss’s office to give the resignation letter? If they ask the reason, what is better explanation? For sure, I will give them 2-3 weeks notice. I want to do it glacefully and professionally.

Thanks for your input.