Welcome! This is what I would do if I were you (this is just an application of the investment order others have posted):
1) Decide how much cash you want to keep in an emergency fund. The common rule of thumb is 3 to 6 months of expenses, but it's a personal choice. I'd say that anything under $1,000 is definitely too little, and anything over 1 year's living expenses is too much -- but there are a lot of good, defensible numbers in that range. You're allowed to let your personal experiences influence your choice here. (FWIW, my assumption for the #s that follow is that you'll decide to keep perhaps $10K or $15K as an emergency fund. If that's a wrong assumption, adjust accordingly.^)
2) Go sign up for your 401k TOMORROW.^^ At the very least, contribute enough to get the maximum match (if there is one). At the very most, contribute $18,500 for the year. (Per my assumption in #1, you can stop contributing to your emergency savings fund for now. So another way to do this might be to look at how much you've been putting into your savings account each month, and set your 401k contribution to that. You're essentially redirecting new savings from cash to tax-advantaged investments.) Make sure the $$ you're putting into your 401k are going into a target-date fund or a nice broad-market index fund. If you're not sure what that means, post your 401k plan's list of investment options here and someone will chime in and help.
3) Take $5,500 from your savings account and go to vanguard.com and open an IRA (Traditional or Roth; do some reading on this to decide, and post here if you want others to weigh in on your choice). Invest that $5,500 in, again, a nice broad-market index fund or a target-date fund. (Congrats, you just maxed out an IRA for 2018! You can't put any more in this account this year, but you can do it again in 2019.)
4) Go find those old 401ks and track down every dollar. (Note: if the balance in these was very low when you left, they may have cashed you out and you may not have realized it.) You'll want to roll all those into a Rollover IRA. Call Vanguard and they'll help you set it up. Again, big dumb index fund or target-date fund.
5) Take what's left from your emergency fund overage and do one of two things: (a) Open another account at Vanguard that's just a plain ol' taxable brokerage account, and buy some more boring index funds.^^^ Hot damn, now you'll have 4 investment accounts (a 401k, two IRAs, and a brokerage account). You laid the groundwork for your long-term financial planning in, like, a week. That's pretty badass. OR (b) Make a one-time student loan payment, all toward principal. At 5.125%, I'd just barely be inclined to go with (a), but if the loan is bugging you, a one-time payment might feel really good. And I'm not above psychological wins.
^ One reason to keep a bigger emergency fund would be if you think you'll leave your current job (and potentially move) in the next year or so. That would be a good reason to keep extra cash on hand.
^^ And figure out if your health plan qualifies you for an HSA (Health Savings Account). If so, that goes in here too.
^^^ For a new account, I think you need $3,000. So if your available amount is less than that, scratch this option.