Creating an RPM package can be complicated. Here is a complete, working RPM Spec file with several things skipped and simplified.

Name:       hello-world
Version:    1
Release:    1
Summary:    Most simple RPM package
License:    FIXME

%description
This is my first RPM package, which does nothing.

%prep
# we have no source, so nothing here

%build
cat > hello-world.sh <<EOF
#!/usr/bin/bash
echo Hello world
EOF

%install
mkdir -p %{buildroot}/usr/bin/
install -m 755 hello-world.sh %{buildroot}/usr/bin/hello-world.sh

%files
/usr/bin/hello-world.sh

%changelog
# let skip this for now

Save this file as hello-world.spec.

Now use these commands:

$ rpmdev-setuptree
$ rpmbuild -ba hello-world.spec

The command rpmdev-setuptree creates several working directories. As those directories are stored permanently in $HOME, this command does not need to be used again.

The command rpmbuild creates the actual rpm package. The output of this command can be similar to:

... [SNIP]
Wrote: /home/mirek/rpmbuild/SRPMS/hello-world-1-1.src.rpm
Wrote: /home/mirek/rpmbuild/RPMS/x86_64/hello-world-1-1.x86_64.rpm
Executing(%clean): /bin/sh -e /var/tmp/rpm-tmp.wgaJzv
+ umask 022
+ cd /home/mirek/rpmbuild/BUILD
+ /usr/bin/rm -rf /home/mirek/rpmbuild/BUILDROOT/hello-world-1-1.x86_64
+ exit 0

The file /home/mirek/rpmbuild/RPMS/x86_64/hello-world-1-1.x86_64.rpm is your first RPM package. It can be installed in the system and tested.