If you haven't learned about "expense ratios" before, and consider yourself to lack experience with investing, I'd recommend Vanguard. It's very hard to make a bad choice at Vanguard, which makes it easier to invest. If you check each mutual fund's expense ratio (which you should do, even at Vanguard) then Schwab and Fidelity are good choices with many low expense ratio funds.
Two reasons why it's important: first, expense ratios consume your money - silently. You won't see it on an account statement, so a fund could take 1% of your money without you knowing. Usually stock market performance is much more significant than the expense ratio - but over time, it matters. Second, total stock market funds (like you mentioned in OP) have low expense ratios because they don't try to spend money beating the market. Usually that doesn't work, and a passive approach of accepting the market return actually beats most funds (80%-90% depending on time frame).
If you don't like your old 401(k) being scattered around and dealing with notices and changes regarding them, you could consolidate by "rolling over" to Vanguard, Schwab or Fidelity. Those big three also have low cost funds - the selections might not be as good elsewhere. Make sure you elect a "trustee to trustee" transfer - getting a check in the mail is bad news, since they have to treat it like a withdrawal and send 20% of your account to the IRS to cover taxes. If they send you a check, you'll have to dig up that 20% from savings and deposit it in your new IRA... and then get your money back next year at tax time. Painful and not worth it: just let your 401(k) go directly into another account. Since Vanguard/Schwab/Fidelity will be receiving your money, and will want to help you, that's the place to fill out a new account form.
