If you're going to do more car mechanicking, invest in some decent tools first.
The one thing that will always be true is that no matter how many tools you have, you will always need one more.
Anyway, start with a decent 8mm allen key. I use one that's set into a socket (Halfords £5) so I can use a breaker bar on it, rather than a ratchet.
A breaker bar has an 18" long handle, that holds a socket. Ratchets are shorter and it's more difficult to get a good leverage on them. Also you might break the ratchet mechanism if you lean on them too heavy.
So... first of all buy a good socket set... then you can add extra accessories to it as you go.
I'd recommend a 3/8" drive for you. 3/8" is a bit smaller than 1/2" and so the tools are lighter, cheaper and more manageable. 1/2" is better for heavy duty stuff but those jobs are probably not for you right now.
Halfords' "Professional" range is pretty good for the money.. so check out a socket set from them.. it should have most of what you need in it (but not the 8mm allen key socket). Also buy their 6 or 8 piece combination spanner set. Now (apart from all the pliers and screwdrivers you will need) you are reasonably well equipped.
The allen key under the sump is anti-clockwise to undo, as you look at it... just like the lid on a bottle of vodka. It's just stiff... so get the biggest breaker bar you can on it and undo it. Keep the allen key/socket straight or it'll slip and round (mash up) the inside of the hex. If yours looks burred, rounded or mashed up, buy a new one (a couple of quid.. soooo much less hassle).
Catch the oil in a pan. It's about 4.5 ltrs in there, so make sure your oil pan is big enough. I use an old 5 ltr oil can with one side cut out (and the lid on, obviously). Make sure your oil can is not a 4 ltr one!
The oil filter, as above. I always use a wrench to remove the oil filter... hammering screwdrivers through it is a bit of a one-shot job. I got a fright once where the filter wouldn't unscrew and the screwdriver ripped the filter off its thread... luckily it just broke the stiction enough so I could remove it with some pliers, otherwise....
A wrench won't wreck the filter.. so if you can't remove it, you can drive the car to somewhere who can. I use a chain wrench .. I have 2 sorts.. one which looks like a chain on a stick and the other has the chain looped through a socket .. for my 145 I think the chain on the stick type works best .. the loop sort is better where you can't really see the filter (my 155 v6) .
That comes off anti-clockwise as you look at it .. same as a bottle of tequila..
Catch the oil that comes out when you undo the filter. It will be about 100ml... not much but enough to make a mess.
To re-fit.... using a new sump bolt as necessary: Don't overtighten the sump plug. Just do it up normal tight. Think how much drama you had to remove it.. it's not going to come unscrewed just like that.
Oil filter: Lube the rubber ring with clean oil and screw it on hand tight..(after you cleaned up the oil drips etc.) Then turn it another 1/4 turn. Don't overdo it.. it'll be you having to remove it later..
Put the new oil on... bung in just about 1/2 ltr first and check for it dripping under the car. Sooner or later you will
put oil in the engine with the sump plug missing...
so always do a quick squirt-and-check.
If you're looking good (no drips) put in another 4 ltrs.
Leave it about 10 mins to settle and check the level. It should be on MAX.
Start the engine, let it idle. The oil light might be on for a few seconds when you start the car but if it goes out in the time it takes for you to think "ohmygodwhatshouldido..?" then it's fine.
Let it idle just for 30 seconds or so... to get the oil into the filter and then leave it for a while to settle again. Tidy up your tools and start clearing up. Count the spanners and sockets and make sure you have the same number you started with...
Check the oil.. the level should be down a bit (it's in the oil filter now) so just top it back up to MAX. You'll use about 4.6 ltrs altogether. Job done.
Old oil.. put it in an oil can and take it to the council tip. They recycle it. Don't bury it in the garden or pour it down the drain. Apart from you can be prosecuted .. it's irresponsible and you'll poison everyone.
Check the beast a week later. If the sump plug looks dry and the oil filter isn't leaking, then they're both not going to come undone.. but have a look when you remember anyway.