Some good mutual fund companies to look at are Fidelity, Vanguard, and T. Rowe. If you're into bonds, you should investigate PIMCO, though I don't like bond funds, because they turn otherwise stable, predictable investments into unstable and unpredictable investments. There's also Royce.
For a detailed review of some Fund Families, take a look at
Kiplinger's Personal Finance, August 2003, p32.
Fidelity probably offers the most choices, hands down. It's the very biggest mutual fund manager in the US. Vanguard offers the lowest expense-ratio funds. Be aware that if you're looking at funds in Vanguard other than their index funds, a lot of them require large initial investments (e.g., $10,000), though they are often well-managed and offer stellar expense-ratios (Vanguard Primecap has 14% return over the past 10years at an expense ratio of just 0.49%). T. Rowe also deserves strong consideration.
Some things you should look at are the diversity of funds offered (how many), the quality of the funds, the expense ratios of the funds, minimum initial investments, and if the mutual fund firm allows you to invest in other Fund Families without penalizing you. Also, when looking at specific funds within a Fund Family, be very wary of funds that have an initial front load or a back-load. I wouldn't ever invest in a fund with a front or back-end load (this i where they charge you a certain percentage just to get into or sell the fund). Also watch out for short-term trading fees. These aren't particularly important if you're dedicated to stick with the fund.
Right now, the most important thing for you is to look at the initial investment required by Fund Families, and if they offer exceptions for Roth IRA's. When inquiring with Fidelity or Vanguard, ask if they will allow smaller initial investments, contingent upon a regular investing schedule.