Yup, I'm with Boydie - REAPER's the way to go.
I know a lot of people here use audacity, but it's like using a hammer when you need a screwdriver: you can probably get the screw in, but only just - and it'll take a lot more effort.
The problem is that the learning curve on a DAW can be almost vertical at first. But it's worst in the first week or so, then after that the slopes are gentle and the view sublime. And the REAPER manual's good, the community's helpful, and there's quite a lot on YouTube. Also, I'm a REAPER guy, so if you get stuck, I'm happy to help.
What kind of instruments are you looking for? Boydie's shout on Computer Music magazine is absolutely on the money, as it contains a lot of synths for almost no cash. But if you're after traditional rather than synthesised instruments, you'll quickly want to move on to something else.
If you are interested in traditional instruments, it can get expensive fast. You will probably want Kontakt 5, the industry-standard sampler made by Native Instruments. It comes with 43GB of sample packs covering guitars, pianos, orchestral, etc. The sounds it comes with are not bad, but are by no means the best. But there are literally thousands of instruments available for purchase, and many of them are unbelievably good. There are also quite a lot of free instruments for Kontakt, many of which are also very good - but you need to pay for Kontakt to use them.
(One thing that confuses a lot of beginners is that there's a free version of Kontakt. But it will only play most instruments for 15 minutes before you need to restart it.)
There's another option for free traditional instruments called soundfonts, but I find they sound bad and setting up a soundfont player is more trouble than it's worth.
If you are interested, Native Instruments is currently having their annual cyber Monday sale - until the 9th. Many other companies are doing the same, so it's a really good time to buy. For example, Kontakt is currently reduced from $399.00 to $199.50. They also have a weird "crossgrade" pricing system where buying certain instruments entitles you to further discounts...
Native instruments sell two bundles which include Kontakt along with synths and sample packs. They have Komplete 9 and its big brother, Komplete 9 Ultimate. They're expensive, but they may be a worthwhile investment... Of course, it depends what you need, and you may be better buying a small number of instruments that are exactly right for you rather than a shitload that are not quite right.
I know you asked for free and online, and I have veered a long way from that - but while there are some interesting developments in free online DAWs, they are not properly useful yet and will not be for some time.