You have two choices really - get an "all-in-one" surround sound set up (active/powered sub woofer that has 6 RCA inputs, and includes 5 speakers that connect to the outputs on the active sub). You simply plug in the multi-RCA output from your DVD to the inputs on the active sub, plug in each speaker (to the sub amps outputs) and away you go. Benefits to these is they are cheap, easy and space saving.

The other option is to get an AV amp. These have a shite load of input on them, including the multichannel inputs for 5.1, along with optical and digital inputs also for 5.1 and DTS, and a host of stereo RCA inputs for CD, Tape, Phono, etc, etc. Also have inputs for video switching aswell (typically in composite and S-Video form, but the more expensive ones have component (RGB phono's) and now, HDMI aswell.....

But with that, you will also need 5 speakers and a powered sub (or a normal sub and another amp - most AV amps do not include an amp channel for subs, only a pre-out) But AV amps do have amping for 5 speakers minimum, and nowadays you can get them to power 6 or 7 speakers for 6.1 and 7.1 systems....

I myself have gone down the AV Amp route and bought an older Sony STR-DE685 AV amp, which should take all I want to input to it (DVD via digi-coax for DTS, SACD via multi-channel input for 5.1, CD via Stereo RCA, VCR via stereo RCA, freeview STB via optical, and a media player of sorts probably via optical) I won't be using the video switching as RGB Scart is of better quality than S-vid (and my AV amp doesn't support component video switching) and composite video (yellow RCA plug) is simply dire and isn't worth the price of the cable!!!!

I now have a lovely set of Mission floor standers to plug into this, along with an Eltax centre, but I still need a pair of Mission rears and a sub....

My TV tho, is currently taking car of the 5.1 soundtrack tho as it has a digital input (and onboard decoder for Dobly Surround) and has surround satellite speakers included, and a built in subwoofer VERY easy indeed