First WELCOME to IIS
Next I'll give my standard answer to all newbies with their first scope. The set of eyepieces you have been given by Bintel is a great start and for the short term there is no need to expand it or to get better lenses.
The things you need to do now to get better views are:
- Use it often and get used to handling it.
- Allow it to cool for at least an hour in the place you are going to view from to allow the mirror to get near ambient temperatures. This will reduce the effects of thermal currents in your tube which degrade views.
- Ensure it is collimated. I would recommend your first purchase to be an Orion Collimating Eyepiece from Bintel. If the scope is not collimated it will never give you great views.
- Allow for bad seeing, if the stars are twinkling then the seeing is bad and you will never see the Cassini Division. It will be a bit like trying to see the bottom of a swimming pool when someone has dived in. This is caused by many factors but lately the jet streams have been over us and messing up the air. On nights like this go for the low power stuff like clusters, etc and avoid trying to get detail on planets and splitting doubles, etc.
- A good barlow should be your other first purchase. I have used the Orion Shorty Plus but now I have a Meade #140 and I prefer it. Pick your poison but make it a good one otherwise forget it.
- Did I mention, use your scope??
- Practice!!
- Join the local astro society and go to their observing sessions. Lots of great experience there for you to plug into directly.
Apart from that enjoy the views.
As for the fast and slow scopes, shorter focal length ones f5 and below are fast and give great wide angle views f8 and above are slow and are more suited to planetary observing where high magnification is important. Our f6 scopes are great middle of the road scopes.
Above all else, keep looking up.