For a simple bar, I'd build a frame and drop a countertop on top. For
my build I used a 25"x6' butchers block countertop. Build the frame out of SPF #2 2x4s, which are dirt cheap so mistakes are no big deal. You don't need much experience to do this. If you've never done any woodworking before, I recommend watching
Steve Ramsey's Woodworking for Mere Mortals. A lot of the channel is aimed at small woodworking projects, rather than the skills you need for a bar build, but I learned a ton from his videos.
I'd recommend two tools to make your life much easier: a decent
mitre saw (you can pick up small ones for just over $100 from big box stores like Home Depot or Lowes), and a
Krieg Jig. With these, you can have it it with 2x4 to make a frame. The mitre saw will help you gets straight cuts and the you can join them together with pocket hole screws using the Krieg Jig.
If you go this route, take some time when you pick out the 2x4s to get nice straight ones by looking down all four sides of the wood for warping. Getting straight lumber will make your life easier. A local lumber yard can be a better bet than the big box stores for supplies.
If your floor isn't perfectly level, when you make the 2x4 frame, make a base that sits on the floor and then make a second identical base that sits on top of that. Shim the second base with some off-cuts so it's level and then screw them together. Then you have a level surface to build the rest of the frame on. Take the time to measure your cuts for the uprights so everything stays level. It's worth taking the time to make an extension to the mitre saw and
rigging up a stop block to keeps your cuts the same.
Whatever you do, I'd also recommend finishing the wood with
Rubio Monocoat Oil Plus 2C. It's not cheap, but it's much easier to work with than polyurethane finishes. However, it won't give you that "shiny" bar top, if that's what you're after.