Get the biggest sump you can manage/fit in your space. It can be an old tank, a rubbermaid bin, even a bucket. It would be best to use a tank though so you can divide it into baffled sections for a skimmer, refugium and return area. You may just want a refugium or maybe just a skimmer but it would be best if you figure that out now so you allow enough space in your sump section to account for your needs. In-sump skimmers will not only need to fit in the length and width of your sump but also the water height that the skimmer sits in is important and unique to each make/model of skimmer.
If you can I would get a drilled tank ("reef ready") or if the tank you have now is not built with tempered glass, you can drill it yourself and get one of the internal overflow boxes on the market. If neither is possible you can run a hang-on overflow. Just know that the hang-ons have some risk for flooding.
If a sump isn't something you want then you can use a hang-on skimmer and/or hang-on refugium. There are manufactured refugiums or you could also use a modified Aquaclear (or similar) HOB filter.
Research the beta and compatible tank mates. Pretty sure small fish are fair game as well as decorative shrimps.
There are lots of "starting up" posts out there. If you just look down the first page you will find quite a few that will give you some ideas. You can also checkout others build threads and see how they went about things.