you're going to get hit with anywhere from $10-20 per transaction.
Thus, buying in with 100 bucks already puts you 10-20% in the hole.
I generally don't buy under $1000 worth of whatever.
Some funds have min limits, too.
And some stocks are cheaper the more you buy. This usually saves a few cents a share once you break 1000 shares, or whatever their big buy figure is.
That said, if you open a fund with someone, chances are they have an interest bearing 'cash reserves' that pays WAY over any savings passbook will.
I'm currently earning 2.35% in my cash reserves, and was over 5% just a few months ago.
So,
Dump a 100 a month into it if you can. If you can dump more, great.
I personally dump 350 into my 401k (175 each me and my company match pre-tax, directly out of my check) and 200 into my roth IRA (direct deposit from my bank each month, post-tax).
roth ira's earn interest free. the only tax you pay is that it is post-income, so it's already been taxed. but, they have limits. Currently, 5k a year max contribution.
the 401k is pre tax now, but when i take it out later at 55+, i will have to pay taxes on it.
basically, it earns interest/investment value on the tax dollars, and then the gov't takes that interest you earned on 'them' for allowing it that way.
and the roth is the other way around, it's your money 100% when you take it out, but you don't earn interst on the taxable extra as they already took their cut before you put it into it in the first place.
If you're not remotely concerned with retirement funds (but you should be) you can always get a simple 'trader' account.
This will be 100% post tax money that you put in, and can take out at will with no age limits. But, you will be hit with capital gains on anything you pull.
I say start an account with no less than 250.
and don't make your first stock purchase until you have 1000 in it.