All that is discussed in the Activation and Licensing part of the Release 8 User Manual. Please read that carefully, for example, the Activation Keys and Serial Numbers topic. Don't skim or skip over anything. Actually read that topic (and other relevant topics, such as installing extensions) from beginning to end. Do what that topic says.
Respectfully, if a pointer to the right topic is not enough, the forum is not the place for more extensive help on how to install or activate. See http://manifold.net/tech/activation_support.shtml for help.
Regarding 9 vs 8:
9 is different than 8. Make sure you read the introductory topics carefully. If you skip the introductory reading and just try to "wing it" as if 9 were 8, you'll encounter lots of totally unnecessary frustration. Read the introductory topics and it falls into place quickly. See also comments in the FAQ comparing 9 to 8, download the free Viewer (everybody should have Viewer... if you're doing anything in GIS it's just plain nuts not to have it in the toolbox), follow along in the first three tutorials in the Videos page.
As to whether you should go with 9 or 8 it depends what you want to do with GIS.
I far prefer 9 because it has way more data sources and formats than 8, and I like the speed. For me, for exploring the world around me and drilling into data to discover what I want to know, 9 is hands down a better tool than 8.
On the other hand, 8 has a seemingly endless roster of pre-built tools and conveniences. It has many more interactive raster tools, although since 8 is limited to relatively small rasters (a 100 MB raster in 8 is a big deal) it's often painful to apply them to the particular image you are working with. That's the same drill as with all the classic GIS packages, which have lots of features but then when you go to use them you discover they are painfully slow on the data sizes that you're working with. What used to be "bigger data" ten years ago is now just routine stuff.
People doing working cartography tend to prefer 8 or a mix of 8 and 9 because 8 still (so far) has more conveniences in Layouts for that, for example, borders showing graticule ticks.
I'm puzzled why you would need to buy a 9 license if you have an 8 license. It's really hard to run out of activations on an 8 license, especially with the Covid special of three free activation keys in 2020.