...I have no credits to spend on anything, barely enough for fuel...:/
First you did not lose any rankings, alliances with factions, reputation etc which can only be acquired by many hours playing the game. The best ship and lots of credits won't buy you that. All you need now is your Asp and some credits. Simple.
The good news.
Go to a Nav Beacon. Do not have any cargo nor pickup any while there and you won't be bothered (Cargo NO materials YES). Wait for a wanted ship to show up and the good guy NPCs start firing at it. Target the ship to make sure that it is wanted. Attacking without verifying can get you wanted! When it has about 20% hull left get in close and fire away with your two puny pulse lasers in the Sidewinder hitting the ship until it explodes. Don't hit the good guys! You will then get the bounty for all the work the NPCs did for you. Instant 4k in credits for the lowest rated ship. Lots of 20k-60k ships if you are patient...but move fast when they show up as the good guy NPCs are very good! Rinse and repeat but don't get too greedy and cash in the Bounties at the system station. Get killed and you lose them.
Hit a good guy and you get wanted with a bounty for 10 minutes. Leave the Nav Beacon and avoid contacts. One could just drop out of supercruise or into a safe USS and wait. After 10 minutes a high wake to another system clears the Bounty. Kill a good guy and you gat a Bounty for say 6 days! Be careful.
Get better ships and weapons and go to more hazardous places as in rez, lo rez, hi rez in rings around planets (but not Combat Zones for now...It's a different game). The higher the risks the better the bounties but the less there will be NPCs to do the work. Do the same and that wanted Anaconda can provide 200k in credits. Don't get greedy! You'll have your 6 mill Asp in a few hours and the 24 mill to A-rate it in a few hours times 4...maybe even sooner depending upon your skills.
Note if you are in your home system allied with the local factions you will lose rep with the ones you shoot. Totally OK to get started as losing rep is about a 20% loss. Think one mission completed for a faction equals 5 combat losses. But get that decent jump drive and move out several systems away so that you don't influence your local rep. If you accidentally kill a good guy NPC you will get a bounty. Easier to deal with when it is NOT in your home system but that solution is in many other threads.
The bad news? As your combat rating goes up so does the skill of your NPC opponents. Get too greedy advancing to say Expert without being able to hold your own in a 1-to-1 fight and you won't be happy. So get a nice comfortable credits reserve and lose a few Sidewinder/Adder ships learning the basic combat skills needed. Then you'll be a lot more confident in that shiny new Asp when the bad guys come your way. No need to spend weeks to get back where you previously were.
You now have the detailed instructions others have mentioned casually in their replies. There are a lot more opportunities now than ever were in the past. This is just one. I have a fleet of the top ships in the PC version worth billions doing it the hard way for two plus years but am awaiting the PS4 release to start it all over again! First I'll go to a Nav Beacon...
Regards