If you're young, I see no issues with putting off 401(k) contributions until you have a decent savings set aside.
Since the OP is young, I'll explain why this thinking is, um, wrong.
If Person A saves and invests 1K per year from age 20 to age 30, then never adds to it, but keeps it invested and
Person B starts saving/investing the same 1K per year in the same manner from age 30 to age 65, who will have more money at age 65?
If you said Person B, you would be incorrect. The earlier you begin, the more time there is for compound interest to work it's miraculous magic. Creating a low-cost, diversified (i.e. Vanguard, NOT Betterment, EJ, et al) portfolio means that both of them get the best possible result, but Person A still wins by a mile with significantly less input and effort.
So, to answer the OP's original question, read the jlcollinsnh blog series about how to start investing. Save a baby EF of 1K and funnel everything else into investments. After studying jlcollins, you will be able to decide where to put the money. I'd go with a Roth because you have the option of withdrawing from it, but the truth is, as a mustachian, you are far less likely to need to tap it than the average consumer sukka, so don't be put off by the article linked upthread. Also, you don't have to fund a Roth to the max at first. A partially filled Roth bucket is far better than none.
Finally, giant kudos to you for killing the SLs and beginning retirement-type savings so soon in life. Your future (probably FIRE) self will be so amazed at how awesome your present self was.