The very best option from the starting gate would be to wait until the next sale on mechbays and use the 50% off discount that PGI normally slaps on those to buy a metric buttload of them. The starting 4 bays will not be enough for you in the long run, and PGI has been pretty stingy about handing out free ones (usually no more than one per event, if that), so if you want to have more than one lance's worth of 'Mechs in your collection, mechbays are the single most important thing you'll spend money on.
I'd recommend you buy enough to give you plenty of room for expansion- 10 would be good, 20 would be better (you need three of each 'Mech you want to master, so unless you only have eyes for one or two 'Mechs, you'll eventually find yourself selling 'Mechs you'd rather keep because your newest toy can't skill up without two more variants). Again,
wait for a sale- mechbays are normally 300MC each, so you could easily run through all of your money just buying places to store 'Mechs if you buy them at their regular price.
Whatever you do with any mc you have left over after that is up to you- if there's a hero variant you like and can afford, then buy that; otherwise... more mechbays, premium time (though you should only get that if/when you plan on using it, since active premium time can't be paused), cosmetics like cockpit items, colors, and camo patterns, and once you master your first chassis you can also spend it to convert surplus 'Mech XP to GXP (which can speed your progression towards unlocking very important modules like Radar Deprivation and Seismic Sensor). If you had twice or three times as much MC on hand I'd recommend buying a mastery bundle (hero, champion, and standard variant plus premium time and other goodies), but mechbays should
definitely be your top priority right now. They are the absolute best value in terms of increasing your enjoyment of the game.
*edit*- That advice about waiting for sales applies to everything that's available for MC, by the way. PGI does sales on all of it, although not usually at the same time. With the holiday season coming up, we're bound to have sales on just about everything by the time New Years' Day rolls around.
One more thing:
never spend MC on consumable items. They're cheap, but it's a total waste.
Edited by PS WrathOfDeadguy, 16 November 2015 - 04:34 PM.