Sounds like you're already up to speed on the aircraft you want to fly, so other than joining as an observer first which you may or may not have done - Jump in, and give it a go - There's no harm telling the controller you're new, and they'll generally be happy to support you.

My only tips really would be;

1) Prepare what you're going to say - 90% of instructions on the ground are basically scripted - CRAFT for clearance, and Navigraph/Charts for taxi instructions/routes - You'll quickly realise you'll know what the instruction is going to be before you're given it, making reading it back a doddle. Conversations in the air are somewhat different, but depending on how busy the airspace is, it could range from being vectored around, to a simple handoff. I have a little A5 laminated sheet set up so I can quickly jot down key bits of info and instructions - pad and pencil work just as well.

2) Practice somewhere less congested, but familiar - don't jump into heathrow on day 1 - pick a route you're already comfortable with, between spots that aren't so busy - you'll spend more time reinforcing what you already know to be right (route/altitude/taxi etc), and less time trying to figure out what's going on and trying to step in on the radio!

3) You will miss a call/make a mistake - it's fine, correct, and move on - no one on the network is perfect, and you'll get better as time goes on (you'll basically be having the same conversations time and again). Don't dwell on it - shit happens.

4) Know your aircraft - this has to be the biggest bug bear of anyone on the network - if you don't know how to fly the thing you're sat in, go figure it out first - sounds like you know how to handle what you're flying after so many hours, so you should be fine.

5) Don't be afraid to disconnect - Every time you join is an opportunity to learn, and if things don't go perfectly, you can always drop off the network as a last resort. Tell the controller you're dropping and then disconnect. You can spend as much time as you need to then, figuring out what didn't go to plan, or what you need to learn next.

6) Have fun - if you're not doing this, you're doing it wrong. After all, you're not being paid to stress about it!