Seconded on the Betterment IRA. My parents started an IRA for me when I was in school, with Vanguard, but I think they have a 10,000 minimum or something now. Betterment has no minimums, although you do pay less in fees once you reach certain thresholds. At $10,000 I think it's about 0.25% per year. They give you a nice basket of Vanguard funds, you can tailor how "risky" you want it to be, and have automatic deductions taken out of your checking/ savings account. We opened my wife's Roth IRA through Betterment.
Since the max an individual can contribute in a calender year to a Roth IRA is $5,500, have $5,500 / 26 put in every other week when you get paid. This is about $211.53.
If you have a spouse, even if he/she has no earnings, you can still put another $5,500 per year into a Roth IRA in _his/her_ name, provided you make at least $11,000 per year (I think...double check that figure)
The 401k max is 18,500 per year for 2018, so you can have up to $711.53 per bi-weekly paycheck deposited into a 401k. Your spouse can also do $18,500, but only if s/he works and makes at least as much as they contribute to the 401k.
I've leaned towards Roth IRA over traditional. My reasoning is that the Roth gives you a known factor- you pay the taxes now, so whatever you earn is tax-free. If tax rates go up in the future, it doesn't matter. Also, I presume that I will be putting into the IRA a lot less than I'll be taking out. So it's a calculation that a little at a higher rate is less taxes than more at a lower rate (assuming my retirement rate _is_ lower, which as I said is not a guarantee.
EDITED Example: if you put $5,500 every year into your IRA (Roth or Traditional), for 40 years, that's $220,000 worth of contributions. If your combined tax bracket is 20%, you had to earn 275,000 to make those 220,000 worth of Roth contributions. But, if your IRA worth after 40 years is $1.2 Million, and you spend all that money through your retirement, even if your combined tax bracket is only 15% you'll still pay $180,000 in taxes on traditional IRA withdrawals (versus only $55,000 if you had the money in a Roth).
Whatever you do, disregard the day-to-day volatility of the market and dollar-cost-average. You're saving for retirement- something only maybe 25% of people do, anyway- so you're already ahead of the game.
Oh, and make sure you buy some Bitcoins, too. I hear they're hot.