OK,  that's a start.   What microphones do you have?   Specific models will let us know where your starting point is.    Also, a bit of info on your current computer will be helpful?   Processor and memory will make a difference in performance.  A solid state drive is great to have, but not an absolute necessity if the rest of the system is powerful enough.  
For a basic system,  you can pick up a Behringer UMC204HD for around $80.   It's a basic audio interface,  has two mic preamps which will let you set up a mic for vocals and a mic for a guitar.  Or you can record a piano in stereo, then add vocals later.   It gives you options.   It also has midi, so you can use your keyboards to control virtual instruments later on.   If you want a better quality unit, you can look at the Motu M2 or M4,  or the Zoom UAC2.  
You need a Digital Audio Workstation (DAW).   Stay away from Audacity.  It's a great editor, and free, but doing multitracking is not it's forte.   I would recommend looking at 
Reaper and 
Cakewalk by Bandlab. Cakewalk is free, Reaper is free to try and $60 for a license. Reaper is a full fledged DAW and can be as basic or as complex as you want. I've been using it for over 5 years. Both come with internal effects called Plug-Ins.
Microphones can be a bit of a rabbit hole.  I would avoid the really cheap USB mics and similar Chinese condensers (like the Fifine and ZINGYOU).   They work, but really aren't quality units.  I've heard quite a few people who end up with non working units.   Their only positive is that they are cheap, usually $25-30.    
I use different ones for my acoustic guitar vs my vocals.   Most of my vocals have been done with a Rode NT1,  although  I've also used a cheap MXL V67g ($80) and an old Studio Projects B3.    I also have a Miktek MK300 that I picked up on sale.  It sounds really good, but I haven't used it for real recordings yet.   For my acoustic (a Taylor 310 dreadnought)  I have Rode M5s and AKG P170s.   Neither is a high dollar mic ($100 or less).    Those are all condensers.  For dynamic mics,  I have Sennheiser e935 and e835.   I prefer them to the Shure SM57 or 58 but that's just a preference.   I also have the 57.    Mics will often go on sale so that one that is normally $300 might be $200, or a $120 mic might be $90.   Patience is a virtue when you are shopping for mics.   Since you already have some,   start with those and then spend time learning about mics.    You don't need a $3000 Neumann to do some decent recording.
Assuming your computer is good enough and you have a decent mic already, you could be recording this weekend for under $100.