OK, first off...do the following:
Hit your Escape key (if in game), and then look at your options.
Look down the list for things like Enable Frameshift drive and Supercruise. These will likely be two different keys, but you CAN put them to one Default key to enable both/either/or, and I am not sure, but I believe the default is 'J'
To increase you cargo, you will need to be docked at a station with OUTFITTING. you can see which station has outfitting by pulling up the SYSTEM map, which can be done either straight from your LEFT (NAVIGATION) panel, or by opening your GALAXY MAP and selecting SYSTEM VIEW from the options there when you mouse over a Star.
The Stations/Outposts will list what services they provide, often things like:
Refueling
Rearming
Ship Yard
Outfitting
Commodities
Not All Stations/outposts provide all services, and some services are not listed at all (like Black Markets).
So, you need a Station/outpost with OUTFITTING.
Your Sidewinder has only a few slots available for cargo bays, and they increase exponentially by their class. Example:
Class 1 - 2 tons
Class 2 - 4 tons
Class 3 - 8 tons
Class 4 - 16 tons
Class 5 - 32 tons
And never seen it, but I am guessing:
Class 6 - 64 tons
So, based on the chart that has been complied buy Commander GunnyHighway:
https://forums.frontier.co.uk/showthread.php?t=73571&p=1215211#post1215211
We see a sidewinder has 1 Class 1 and 2 class 2 internal components.
Your Discovery Scanner is in a Class 1, your Shield Generator is in a Class 2 and your cargo is in a class 2 (4 tons)
You COULD sell your Discovery Scanner and you Class 2 Shield, buy a Class 1 shield and another Class 2 Cargo Bay getting you another 4 tons of Cargo space for 8 tons total.
So again, Class (think Size) is obviously limited to that Class. Class 1, nothing but 1's, Class 2, Can be 1's or 2's. Class 3's, a 1, a 2 or a 3, and so on.
I hope this helps!
And welcome to the Galaxy!