Here are my tips. Pick a kit you are interested in, but try and work on a smaller one (most 1/144 scale) or more simple kit with fewer parts. Heck, you may even want to practice on one of those old and ugly kits from the 80's. Quality of kits are something along the lines of HG (High Grade), RG (Real Grade), MG (Master Grade) and PG (Perfect Grade). A larger scale kit does not always mean more detailed or complicated, this is why there are grades. You will probably want a 1/144 or 1/100 scale either without any kind of grade designation, or at most one labeled HG.
Do not worry about painting, or especially modding and customizing early on. In fact pick a kit that is cheap and you will not mind destroying, because I guarantee your first few kits will have anywhere from a small hiccup to a full blown kit destroying mistake. Mistakes even to people who have been doing this for a while. The difference is you will eventually have the experience to fix or work around those mistakes. It is all part of the process.
For tools make sure you have side cutters to get the parts off the runners, and an exacto knife to scrape away the bulk of the leftovers from your side cutter. You will also want a bunch of sandpaper - for simplicity's sake, 200-300 grade to even out the sprue cut marks, and 400 and 600 grit to smooth it out, slowly working your way up to the finer grit. Do not worry about getting the sanding perfect. This will be a throwaway kit and you just want to work on the techniques. Model glue is something you will want to try working with as well. If you use too much it will melt the plastic. Just a dab on the connections should be all you need in most cases. It is slow to meld the parts together so you can still fix mistakes early.
Your first kit is for learning the technical process. Just work on slowly putting it together and sanding down the rough parts. Painting and customizing are where the real timesink is, so if the initial building process is a bit much for you, then the hobby may not be right for you. After you get that first one put together, do it again on another super cheap kit. Once you get the basics, then find a kit you really like, or one from the next grade up. Get a few kits under your belt before any kind of customizing. Then revisit your first few kits to work on panel lines, painting, topcoats and so on. This way, if you mess up a paint job, its on a junky kit you do not care about.