Hi DaiandGems and welcome to UKAPS!
Firstly you need to establish what your goals are.
What's your budget, how much time do you want to spend maintaining, what types of plants do you want to grow, what kind of aquascape are you after?
These are rhetorical questions so don't feel you need to answer them right now, but think about exactly what you want. My best advice is to take a look around at tanks and aquascapes you like and go from there.
Most (not all) 'decent' 'scapes you'll see have CO2 injection. It's not essential by any means but non-CO2 requires lots of patience; something that newcomers to the planted tank hobby may struggle with, as the enthusiasm sometimes isn't reflected by the progression of the aquarium.
If non-CO2 is definitely for you then as mentioned check out the low-tech / El natural section of the forum. Non-CO2 is much less maintenance (minimal water changes) but growth is around 10x slower.
If you wish to go down the CO2 injection route then consider a pressurised system. These are more expensive but are the most reliable. Poor/fluctuating CO2 results in algae.
Another consideration is to go down the liquid carbon route. This almost half way between CO2 and non-CO2. In a 90 litre tank 500ml of liquid carbon would last around 3 to 4 months and cost around £10. You get nice growth rates without the short-term expense of CO2 injection, but it is toxic if overdosed and a few plants are sensitive to it.
With 2 x T8s you can grow most plants nicely. As a beginner I'd stick with what you have.
The Fluval 205 is a good filter and maybe ample for your needs. However, we often suggest a 10x guideline with flow rates i.e. a 90 litre tank should have a 900lph filter. I'd consider a Fluval 305 or 405 personally, or another manufacturer with similar flow rate.
You also need to consider substrate and nutrient dosing (fertilisers).
Take a look through some of the UKAPS articles on the main site, as well as browsing through the pinned topics and journals. These may throw up more questions than answers but we're here to help, especially if we can see you've put some effort into researching a bit before asking said questions.
😀
All the best,
George