If you're limited to the online function (no one to practice with locally) here is what I would suggest.
Step 1: Play against the computers. Are they great practice for playing real people? No, but they are definitely a start. Work your way up through the levels, and once you can consistently top level 8 and 9, you're probably ready to move on
Step 2: If you can afford it/find it, play against amiibos. Unlike the computers, amiibos will learn from your play patterns, and therefore offer something noticeably closer to playing against a real person. I do strongly suggest getting the amiibo of your main to play against, if only because this helps with avoiding translation issues between characters (for example, I had a DK amiibo that always tried to recover too low to actually survive, possibly because I played against it almost exclusively with Wii Fit Trainer and would recover from very low). The amiibo will also end up with better stats than a standard character, which can be a decent handicap to work against so long as you aren't at a level that focuses on learning combos yet. I'd continue this until you feel ready to move on, but sadly there's no great cutoff for when this should end because the amiibo will continue to improve and adapt indefinitely, just have to use your best judgement.
Step 3: Try your luck at For Fun. This mode often (but not always) seems to have easier matches, both due to the more random nature of the mode and because players either aren't as good or aren't as focused on winning. You'll still get super hard matches on occasion, but not very often, and it's a good place to stretch out and test yourself against real people. Also, if you for some unimaginable reason care about your win/loss record, For Fun doesn't affect it, so no pressure there. If you stop feeling challenged by this mode, or simply feel like your ready to move to a less random environment, it's time to move on.
Step 4: Hop over to For Glory. If you're doing at least moderately well, congrats, you got as far as your asking for with this post.
BONUS STEP: At any given level (but particularly Step 3 onward) try watching a few videos of tournament players using your character of choice, as well as finding tutorials and combo threads. You don't need to be perfect with these, but they can often teach you new tricks to rack up damage or finish stocks more effectively, as well as making your neutral game safer or more aggressive.