Welcome to the Forum!
This isn't going to be an easy one because you say you don't know where to start trying to fix the bike.....and it's your first so the fact that you're here asking for help we'll go with a general overview of the problem and hopefully give you a good starting point to digging in.
The most common cause of No-Start is fuel related. If the bike sits outside in the rain....the tank has internal drain tubes around the filler cap that can let water in if the tubes have broken and there was a recall years ago for the 2nd Gen tanks that had this issue. Water in the fuel means the engine just won't run...and cranking it trying to start is useless until you get good fuel into the system...if it's contaminated. The fuel inlet strainers can and will clog up as will the High Pressure Filter...but these issues normally won't stop it from running...just hurt the power available. The fuel pump should run for 3 seconds every time you cycle the ignition key...open the fuel cap and listen carefully for that 3 second pump run and if it's not doing this....you likely have found at least one of your problems.
Be sure the battery is fully charged before trying to start the bike again....a low battery will let the engine crank but can't spin it fast enough to actually run....so continued slow cranking does nothing good and for SURE can foul the plugs in just a second or two of slow cranking without starting. The engine should start without touching anything besides the clutch lever disengaged and hitting the starter button. If it doesn't fire RIGHT up.....stop cranking, wind the throttle fully open then try again while holding the throttle fully open. This gives it a chance to clear out the fuel fouling and might start but be ready to let go of the throttle once it barks.
I know that many new riders just want to get a bike and RIDE!....but unless you're rolling in cash and have a local mechanic to do all of your work....YOU need to become familiar with the bike and become proficient in fixing things to be able to ride. There is a difference between a Rider and a Consumer riding a bike....a Rider does his/her own work and man does this free up a LOT of money for you! Invest in the tools you need rather than paying someone else and then YOU have what you need next time to fix the bike.
Looking at the bike in the pic....that thing has lived a HARD life! No telling how many things are not done right in its' past that might be contributing to the problem at hand...but buying used always comes with the Previous Owner damage or possible unwitting sabotage so keep this in mind when troubleshooting.
In general terms: you need fuel, compression and spark for the motor to run and I'd troubleshoot in that order. Pull the sparkplugs and see if they're wet? If so...then you're getting something out of the injectors and either it won't burn or one of the other components is missing. The electronic ignition system is able to make a very high voltage spark...but that spark lacks current flow so the amount of actual heat available is very low...so the plugs can and will foul quickly when trying to start...especially if it's cranking slowly. Having had a fouled plug before means that something isn't right and you might start with a fresh set of plugs along with the battery charged first. Riding in the rain can and will foul the front plug when water sprays on the cylinder and the plug well drain hole is clogged which ends up submerging the plug which shorts it out. Make sure those drain holes are open, plugs are clean, battery is charged and fuel in the tank is fresh...then see what you have. Once you get it started again...check that the charging system is working properly because if it's not...riding will slowly kill the battery and you'll be stuck again.
Welcome to the Wonderful World of Biking! There are lots of helpful people here willing to assist you....start with the basics I've outlined above and see how you get on...then report back and we'll go from there. Best of luck!